grep.h: remove unused "regex_t regexp" from grep_opt

This "regex_t" in grep_opt has not been used since
f9b9faf (builtin-grep: allow more than one patterns., 2006-05-02),
we still use a "regex_t" for compiling regexes, but that's in the
"grep_pat" struct".

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

grep.h

@@ -136,7 +136,6 @@ struct grep_opt {
 
 	const char *prefix;
 	int prefix_length;
-	regex_t regexp;
 	int linenum;
 	int columnnum;
 	int invert;