[BUG #6169] symbol("/").pfix is never called, so removed

- because regexp literals are already parsed by the Tokenizer
- and returned as e.g. {type: regexp, source: '/xyz/g'}.
[BUG #6169] bug in parser resulted in wrong syntax tree

- there was a relict prefix() call for "/" in regexp expressions
- which was never used, as regexp literals are completely parsed in the
  Tokenizer
- but botched the precedence rules for infix (left-assoc.) operators

- this showed when activating the Reducer for the entire tree of a class
  which does constant folding in math ops