grep/pcre2: drop needless assignment to NULL

Remove a redundant assignment of pcre2_compile_context dating back to
my 94da9193a6 (grep: add support for PCRE v2, 2017-06-01).

In create_grep_pat() we xcalloc() the "grep_pat" struct, so there's no
need to NULL out individual members here.

I think this was probably something left over from an earlier
development version of mine.

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

grep.c

@@ -373,8 +373,6 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
 	int patinforet;
 	size_t jitsizearg;
 
-	p->pcre2_compile_context = NULL;
-
 	/* pcre2_global_context is initialized in append_grep_pattern */
 	if (opt->ignore_case) {
 		if (!opt->ignore_locale && has_non_ascii(p->pattern)) {