* lisp/character-fold.el: Remove special case-folding support

(character-fold-to-regexp): Remove special code for
case-folding.  Char-fold search still respects the
`case-fold-search' variable (i.e., f matches F).  This only
removes the code that was added to ensure that f also matched
all chars that F matched.  For instance, after this commit, f
no longer matches 𝔽.

This was necessary because the logic created a regexp with
2^(length of the string) redundant paths.  So, when a very
long string "almost" matched, Emacs took a very long time to
figure out that it didn't.  This became particularly relevant
because isearch's lazy-highlight does a search bounded by (1-
match-end) (which, in most circumstances, is a search that
almost matches).  A recipe for this can be found in bug#22090.

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