remove pattern matching specs

[BillWagner]

We merged dotnet/csharpstandard#757, which adds a new clause to the standard for pattern matching.

• remove the feature specs for Pattern matching in C# 7.n
• Combine TOC sections for 7.0 - 7.3. It looked odd now that all the 7.1 specs have been merged.
• Update a lot of links.

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📄 File	🔗 Preview link
docs/csharp/advanced-topics/reflection-and-attributes/index.md	Attributes
docs/csharp/asynchronous-programming/async-return-types.md	Async return types (C#)
docs/csharp/fundamentals/exceptions/creating-and-throwing-exceptions.md	Creating and Throwing Exceptions
docs/csharp/fundamentals/exceptions/exception-handling.md	Exception Handling (C# Programming Guide)
docs/csharp/language-reference/builtin-types/enum.md	Enumeration types (C# reference)
docs/csharp/language-reference/builtin-types/nullable-value-types.md	Nullable value types (C# reference)
docs/csharp/language-reference/builtin-types/unmanaged-types.md	Unmanaged types (C# reference)
docs/csharp/language-reference/compiler-messages/cs0178.md	Compiler Error CS0178
docs/csharp/language-reference/keywords/method-parameters.md	Method Parameters (C# Reference)
docs/csharp/language-reference/keywords/new-constraint.md	new constraint (C# Reference)
docs/csharp/language-reference/keywords/new-modifier.md	new modifier (C# Reference)
docs/csharp/language-reference/keywords/override.md	override (C# reference)
docs/csharp/language-reference/operators/arithmetic-operators.md	Arithmetic operators (C# reference)
docs/csharp/language-reference/operators/await.md	await operator - asynchronously await for a task to complete
docs/csharp/language-reference/operators/bitwise-and-shift-operators.md	Bitwise and shift operators (C# reference)
docs/csharp/language-reference/operators/boolean-logical-operators.md	Boolean logical operators - AND, OR, NOT, XOR
docs/csharp/language-reference/operators/equality-operators.md	Equality operators - test if two objects are equal or not
docs/csharp/language-reference/operators/index.md	C# operators and expressions
docs/csharp/language-reference/operators/lambda-operator.md	Lambda expression (=>) operator defines a lambda expression
docs/csharp/language-reference/operators/member-access-operators.md	Member access operators and expressions - the dot, indexer, and invocation operators.
docs/csharp/language-reference/operators/operator-overloading.md	Operator overloading - predefined unary, arithmetic, equality and comparison operators
docs/csharp/language-reference/operators/pointer-related-operators.md	Pointer related operators - take the address of variables, dereference storage locations, and access memory locations
docs/csharp/language-reference/operators/subtraction-operator.md	- and -= operators - subtraction (minus)
docs/csharp/language-reference/operators/true-false-operators.md	true and false operators - treat your objects as a Boolean value
docs/csharp/language-reference/operators/type-testing-and-cast.md	Type-testing operators and cast expressions - is, as, typeof and casts
docs/csharp/language-reference/statements/checked-and-unchecked.md	checked and unchecked statements (C# reference)
docs/csharp/language-reference/statements/fixed.md	fixed statement - safely access memory underlying a variable
docs/csharp/language-reference/statements/iteration-statements.md	Iteration statements - for, foreach, do, and while
docs/csharp/language-reference/statements/jump-statements.md	Jump statements - break, continue, return, and goto
docs/csharp/language-reference/statements/selection-statements.md	Selection statements - if, else, and switch

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[BillWagner]

closing and reopening for a fresh build. Here come the warnings.

[CamSoper]

LGTM