st/theme: Remove entry from files_by_stylesheet after emitting signal

st/theme: Remove entry from files_by_stylesheet after emitting signal

Since e06109c23ce2a6dd53d32c26a89ab46a952d7373 we keep old theme nodes
valid during the emission of the "custom-stylesheets-changed" signal.

It turns out that we might still look up the file of a stylesheet using
the files_by_stylesheet hashtable during the emission of that signal,
causing a crash because the assertion in _st_theme_resolve_url() fails.

So fix that and remove the stylesheet entry from the files_by_stylesheet
hashtable after emitting the "custom-stylesheets-changed" signal. And to
be consistent, also remove the entry from the stylesheets_by_file
hashtable after emitting the signal.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2757