grl-metadata-key: fix introspection
grl-metadata-key: fix introspection

The following commit has break gobject-introspection of Grilo after a
comment with a line break was added to deprecate a constant:

    608eba2 "core: deprecate mb-album-id metadata-key and related
    API" in 2019-07-15 by Victor Toso <me@victortoso.com>

The issue is being tracked at [0] but a simple workaround is to remove
the line break.

[0] https://gitlab.gnome.org/GNOME/gobject-introspection/issues/294

Before this patch, grl-metadata-keys with value from 53 to 62 were
lost in introspection, that is, from GRL_METADATA_KEY_MB_TRACK_ID till
the recent GRL_METADATA_KEY_MB_RELEASE_GROUP_ID

grilofw/grilo,jasuarez/grilo,grilofw/grilo,jasuarez/grilo,GNOME/grilo,grilofw/grilo,jasuarez/grilo,GNOME/grilo