Revert "meson: fail build when encountering unused variables"

This reverts commit e1e1ef09f9.

This change was misguided. The warning is enough during development and will
get fixed, but turning this into a hard failure just makes WIP harder. Also, a
hard error increases the likelyhood of a build failure in scenarios where
somebody is disabling components (as seen e.g. in
ba8801a076). We already are not very good at
keeping our codebase compile correctly as it ages, because of changes in
compilers and dependencies, and we should not go out of our way to increase the
probability of failure. Such scenarios are painful for downstream builds.

meson.build

@@ -405,7 +405,6 @@ possible_common_cc_flags = [
         '-Werror=shift-overflow=2',
         '-Werror=strict-flex-arrays',
         '-Werror=undef',
-        '-Werror=unused-variable',
         '-Wfloat-equal',
         # gperf prevents us from enabling this because it does not emit fallthrough
         # attribute with clang.