build: Don't disable checks in release builds

Both Clutter and Cogl use g_return(_val)_if_fail() to safeguard
introspected API. Release builds were dropping these checks, which could
result in a much more crashy experience, especially when considering
extensions, but also due to bugs in the shell code itself.

This won't affect any major distro, because they all use "plain" builds.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2930>

clutter/meson.build

@@ -21,7 +21,6 @@ if get_option('debug')
 elif buildtype != 'plain'
   clutter_debug_c_args += [
     '-DG_DISABLE_ASSERT',
-    '-DG_DISABLE_CHECKS',
     '-DG_DISABLE_CAST_CHECKS',
   ]
 endif

cogl/meson.build

@@ -106,7 +106,6 @@ if get_option('debug')
   ]
 elif buildtype != 'plain'
   cogl_debug_c_args += [
-    '-DG_DISABLE_CHECKS',
     '-DG_DISABLE_CAST_CHECKS'
   ]
 endif