grep: remove redundant and verbose re-assignments to 0

Remove the redundant re-assignments of the fixed/pcre1/pcre2 fields to
zero right after the entire struct has been set to zero via
memset(...).

See an earlier related cleanup commit e0b9f8ae09 ("grep: remove
redundant regflags assignments", 2017-05-25) for an explanation of why
the code was structured like this to begin with.

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

grep.c

@@ -174,28 +174,18 @@ static void grep_set_pattern_type_option(enum grep_pattern_type pattern_type, st
 		/* fall through */
 
 	case GREP_PATTERN_TYPE_BRE:
-		opt->fixed = 0;
-		opt->pcre1 = 0;
-		opt->pcre2 = 0;
 		break;
 
 	case GREP_PATTERN_TYPE_ERE:
-		opt->fixed = 0;
-		opt->pcre1 = 0;
-		opt->pcre2 = 0;
 		opt->regflags |= REG_EXTENDED;
 		break;
 
 	case GREP_PATTERN_TYPE_FIXED:
 		opt->fixed = 1;
-		opt->pcre1 = 0;
-		opt->pcre2 = 0;
 		break;
 
 	case GREP_PATTERN_TYPE_PCRE:
-		opt->fixed = 0;
 #ifdef USE_LIBPCRE2
-		opt->pcre1 = 0;
 		opt->pcre2 = 1;
 #else
 		/*
@@ -205,7 +195,6 @@ static void grep_set_pattern_type_option(enum grep_pattern_type pattern_type, st
 		 * "cannot use Perl-compatible regexes[...]".
 		 */
 		opt->pcre1 = 1;
-		opt->pcre2 = 0;
 #endif
 		break;
 	}