Fix st texture cache crash

This is the description of how I think the crash happened from the commit that fixes it:

If the icon surface of a window got updated and its type or format no
longer match what we expect, st_texture_cache_reset_texture() might
overwrite the existing image in the bind with a new image, while still
keeping the weak ref on the old image. Due to this the old image might
trigger a st_texture_cache_on_pixbuf_notify() after the bind has already
been freed by g_signal_handlers_destroy() in the bind source. While this
usually would remove the weak ref, the weak ref it tries to remove is
on the new image, not the old one. The call to
st_texture_cache_on_pixbuf_notify() then tries to read the already
free'd memory from the bind which causes the cast to G_OBJECT to fail,
resulting in the crash.

I've decided to also revert 87558efb, because it did not fix this crash and needlessly complicated the code.

Fixes #3785 (closed)