Remove escape rule

We no longer represent characters via escape sequences. These can be
confused with the literal two bytes of backslash followed by a
character. See the "common productions" list for how these are now
referred to.

Author: ehuss

src/notation.md

@@ -9,7 +9,6 @@ The following notations are used by the *Lexer* and *Syntax* grammar snippets:
 | CAPITAL           | KW_IF, INTEGER_LITERAL        | A token produced by the lexer             |
 | _ItalicCamelCase_ | _LetStatement_, _Item_        | A syntactical production                  |
 | `string`          | `x`, `while`, `*`             | The exact character(s)                    |
-| \\x               | \\n, \\r, \\t, \\0            | The character represented by this escape  |
 | x<sup>?</sup>     | `pub`<sup>?</sup>             | An optional item                          |
 | x<sup>\*</sup>    | _OuterAttribute_<sup>\*</sup> | 0 or more of x                            |
 | x<sup>+</sup>     |  _MacroMatch_<sup>+</sup>     | 1 or more of x                            |