Remove dead code.

The PPSS cache can never be inserted into in the middle, so testing whether
to insert into the middle is unnecessary.

rainbow-delimiters-syntax-ppss-run adds all the intermediate results
to the front of the cache, so it always leaves the cache in a state where
the elements are always exactly
rainbow-delimiters-parse-partial-sexp-cache-max-span apart, so there's no
gap bigger than rainbow-delimiters-parse-partial-sexp-cache-max-span to
insert into.

The before change hook always removes all elements until before the point
of the change, leaving no gap either.

The only way for a gap bigger than
rainbow-delimiters-parse-partial-sexp-cache-max-span to arise is the user
changing that variable, but that variable an implementation detail, so
worrying about it changing is like worrying about
(fset #'rainbow-delimiters-syntax-ppss-run #'ignore).

All in all, it serves only to make the code more complicated. So remove it.
Use double hyphen to indicate internal symbols.
Use #' to quote function names.
Reindent after cf61a3aee6.