meson: Don't run glib-compile-schemas

We don't install anything to $datadir/glib-2.0/schemas right now,
on NixOS which install packages in separate prefixes, try running
glib-compile-schemas will fail with the following:

```
Running custom install script '/nix/store/...-glib-2.77.1-dev/bin/glib-compile-schemas /nix/store/...-loupe-45.beta/share/glib-2.0/schemas'
--- stdout ---

--- stderr ---
Error opening directory “/nix/store/...-loupe-45.beta/share/glib-2.0/schemas”: No such file or directory

FAILED: install script '/nix/store/...-glib-2.77.1-dev/bin/glib-compile-schemas /nix/store/...-loupe-45.beta/share/glib-2.0/schemas' exit code 1, stopped
FAILED: meson-internal__install
/nix/store/...-meson-1.1.1/bin/meson install --no-rebuild
ninja: build stopped: subcommand failed.
error: builder for '/nix/store/...-loupe-45.beta.drv' failed with exit code 1;
```

meson.build

@@ -65,7 +65,6 @@ endif
 
 gnome.post_install(
   gtk_update_icon_cache: true,
-  glib_compile_schemas: true,
   update_desktop_database: true,
 )