Get rid of re_set_whitespace_regexp

Get rid of re_set_whitespace_regexp

* src/regex.h (re_set_whitespace_regexp): Delete.
(re_compile_pattern): Add whitespace_regexp argument #ifdef emacs.

* src/regex.c (re_set_whitespace_regexp, whitespace_regexp): Delete.
(regex_compile): Add whitespace_regexp argument #ifdef emacs and wrap
whitespace_regexp-related code in an #ifdef emacs so it’s compiled out
unless building Emacs.
(re_compile_pattern): Pass whitespace_regexp argument to regex_compile

* src/search.c (compile_pattern_1): Don’t use re_set_whitespace_regexp,
pass the regex as argument to re_compile_pattern instead.