grep: prefer UNUSED to MAYBE_UNUSED for pcre allocators

We provide custom malloc/free callbacks for the pcre library to use.
Those take an extra "data" parameter, but we don't use it. Back when
these were added in 513f2b0 (grep: make PCRE2 aware of custom
allocator, 2019-10-16), we only had MAYBE_UNUSED.

But these days we have UNUSED, which we should prefer, as it will
let the compiler inform us if the code changes to actually use the
parameters.

I also moved the annotations to come after the variable name, which is
how we typically spell it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -245,7 +245,7 @@ static int is_fixed(const char *s, size_t len)
 #ifdef USE_LIBPCRE2
 #define GREP_PCRE2_DEBUG_MALLOC 0
 
-static void *pcre2_malloc(PCRE2_SIZE size, MAYBE_UNUSED void *memory_data)
+static void *pcre2_malloc(PCRE2_SIZE size, void *memory_data UNUSED)
 {
 	void *pointer = malloc(size);
 #if GREP_PCRE2_DEBUG_MALLOC
@@ -255,7 +255,7 @@ static void *pcre2_malloc(PCRE2_SIZE size, MAYBE_UNUSED void *memory_data)
 	return pointer;
 }
 
-static void pcre2_free(void *pointer, MAYBE_UNUSED void *memory_data)
+static void pcre2_free(void *pointer, void *memory_data UNUSED)
 {
 #if GREP_PCRE2_DEBUG_MALLOC
 	static int count = 1;