grep: prefer UNUSED to MAYBE_UNUSED for pcre allocators

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 513f2b0bbd (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>