bpo-40204: Fix duplicated productionlist names in the doc (GH-21900)

Sphinx 3 disallows having more than one productionlist markup with
the same name. Simply remove names in this case, since names are not
shown anyway. For example, fix the Sphinx 3 warning:

Doc/reference/introduction.rst:96: duplicate token description
of *:name, other instance in reference/expressions

Author: vstinner

Doc/library/string.rst

@@ -308,7 +308,7 @@ non-empty format specification typically modifies the result.
 
 The general form of a *standard format specifier* is:
 
-.. productionlist:: sf
+.. productionlist::
    format_spec: [[`fill`]`align`][`sign`][#][0][`width`][`grouping_option`][.`precision`][`type`]
    fill: <any character>
    align: "<" | ">" | "=" | "^"

Doc/reference/introduction.rst

@@ -93,7 +93,7 @@ Notation
 The descriptions of lexical analysis and syntax use a modified BNF grammar
 notation.  This uses the following style of definition:
 
-.. productionlist:: *
+.. productionlist::
    name: `lc_letter` (`lc_letter` | "_")*
    lc_letter: "a"..."z"