build: fix warning about using -Wextra flag

build: fix warning about using -Wextra flag

Each build, meson would issue a warning reporting that the
"warning_level" setting should be used in place of adding -Wextra
directly to our build commands. Testing with meson 0.61 shows that the
only difference for gcc and clang builds between warning levels 1 and
2 is the addition of -Wextra, so we can remove the warning by deleting
our explicit set of Wextra and changing the build defaults to
warning_level 2.

Fixes: 524a0d5d66b9 ("build: enable extra warnings with meson")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>