grep.h: remove unused grep_threads_ok() declaration

This function was removed in 0579f91dd7 (grep: enable threading with
-p and -W using lazy attribute lookup, 2011-12-12), but not its
corresponding *.h declaration.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -207,7 +207,6 @@ void grep_source_load_driver(struct grep_source *gs,
 int grep_source(struct grep_opt *opt, struct grep_source *gs);
 
 struct grep_opt *grep_opt_dup(const struct grep_opt *opt);
-int grep_threads_ok(const struct grep_opt *opt);
 
 /*
  * Mutex used around access to the attributes machinery if