core/namespace: s/normalize_mounts()/drop_unused_mounts()

core/namespace: s/normalize_mounts()/drop_unused_mounts()

Rename the normalize_mounts() helper to drop_unused_mounts. All the
helpers called in there get rid of mounts that are unused for a variety
of reasons. And whereas the helpers are aptly prefixed with "drop" the
overall helper isn't and instead uses "normalize".

Make it more obvious what the helper actually does by renaming it from
normalize_mounts() to drop_unused_mounts(). Readers of code calling this
helper will immediately see that it will get rid of unused mounts.

Link: https://github.com/systemd/systemd/issues/22206