tools: remove no-useless-regex-char-class-escape

The `no-useless-regex-char-class-escape` custom lint rule was introduced
as a less aggressive alternative to some enhancements that were
introduced into ESLint. Those enhancements were blocking us from
updating ESLint. However, they have since been relaxed and the custom
rule is no longer needed. Remove it.

Author: Trott

.eslintrc

@@ -128,7 +128,6 @@ rules:
   assert-fail-single-argument: 2
   assert-throws-arguments: [2, { requireTwo: false }]
   new-with-error: [2, Error, RangeError, TypeError, SyntaxError, ReferenceError]
-  no-useless-regex-char-class-escape: [2, { override: ['[', ']'] }]
 
 # Global scoped method and vars
 globals: