meson: use warning_level=2 by default

Let's bump up the warning level, and not add by -Wextra by hand. This is the
approach recommended by meson. The idea is that all projects should be as
similar as possible to make it easier for users to switch between projects.

meson.build

@@ -8,6 +8,7 @@ project('systemd', 'c',
                 'prefix=/usr',
                 'sysconfdir=/etc',
                 'localstatedir=/var',
+                'warning_level=2',
         ],
         meson_version : '>= 0.46',
        )
@@ -323,7 +324,6 @@ elif want_fuzzbuzz
 endif
 
 possible_cc_flags = [
-        '-Wextra',
         '-Werror=undef',
         '-Wlogical-op',
         '-Wmissing-include-dirs',