Remove obsolete and overly restrictive rule

The purpose of this rule is not clear.
Private methods should be considered an implementation detail.
Hoisting of nested functions was not defined prior to ES2015 and generally discouraged, which makes this rule problematic.
@DrewML assumed it was to help produce cleaner stack traces without `(anonymous)` function names, but since ES2015 function names are derived for function expressions automatically, so if that was the original purpose of the rule, that no longer is a problem.

Author: Vinai

guides/v2.2/coding-standards/technical-guidelines.md

@@ -634,9 +634,7 @@ We are reviewing this section and will publish it soon.
 
 10.5.5. Modules MUST NOT have external side effects.
 
-10.5.6. Function declarations MUST be used for private functions instead of function expressions.
-
-10.5.7. Re-declaration of function names MUST NOT be used.
+10.5.6. Re-declaration of function names MUST NOT be used.
 
 ## 11. Testing