Remove the rustc checket for Rust.

Between #947, #1114, and #1112, this checker is too controversial to
maintain in syntastic.  Please consider using it as an external checker
if you need it:

https://github.com/scrooloose/syntastic/wiki/Syntax-Checker-Guide#external

Author: lcd047

README.markdown

@@ -41,10 +41,10 @@ Elixir, Erlang, eRuby, Fortran, Gentoo metadata, GLSL, Go, Haml, Haskell, Haxe,
 Handlebars, HSS, HTML, Java, JavaScript, JSON, JSX, LESS, Lex, Limbo, LISP,
 LLVM intermediate language, Lua, MATLAB, NASM, Objective-C, Objective-C++,
 OCaml, Perl, Perl POD, PHP, gettext Portable Object, OS X and iOS property
-lists, Puppet, Python, Racket, R, reStructuredText, Ruby, Rust, SASS/SCSS,
-Scala, Slim, Tcl, TeX, Texinfo, Twig, TypeScript, Vala, Verilog, VHDL, VimL,
-xHtml, XML, XSLT, YACC, YAML, z80, Zope page templates, and zsh.  See the
-[wiki][3] for details about the corresponding supported checkers.
+lists, Puppet, Python, Racket, R, reStructuredText, Ruby, SASS/SCSS, Scala,
+Slim, Tcl, TeX, Texinfo, Twig, TypeScript, Vala, Verilog, VHDL, VimL, xHtml,
+XML, XSLT, YACC, YAML, z80, Zope page templates, and zsh.  See the [wiki][3]
+for details about the corresponding supported checkers.
 
 Below is a screenshot showing the methods that Syntastic uses to display syntax
 errors.  Note that, in practise, you will only have a subset of these methods

plugin/syntastic.vim

@@ -19,7 +19,7 @@ if has('reltime')
     lockvar! g:syntastic_start
 endif
 
-let g:syntastic_version = '3.4.0-83'
+let g:syntastic_version = '3.4.0-84'
 lockvar g:syntastic_version
 
 " Sanity checks {{{1

plugin/syntastic/registry.vim

@@ -66,7 +66,6 @@ let s:defaultCheckers = {
         \ 'racket':      ['racket'],
         \ 'rst':         ['rst2pseudoxml'],
         \ 'ruby':        ['mri'],
-        \ 'rust':        ['rustc'],
         \ 'sass':        ['sass'],
         \ 'scala':       ['fsc', 'scalac'],
         \ 'scss':        ['sass', 'scss_lint'],