vinyl: fix race between compaction and gc of dropped LSM

An LSM tree (space index, that is) can be dropped while compaction is in
progress for it. In this case compaction will still commit the new run
to vylog upon completion. This usually works fine, but not if gc has
already purged all the information about the dropped LSM tree from vylog
by that time, in which case an attempt to commit the new run will result
in permanently broken vylog (because compaction will write vylog records
for a non-existing object):

ER_INVALID_VYLOG_FILE: Invalid VYLOG file: Slice 13 deleted but not registered

To prevent this from happening, let's make compaction silently drop the
new run without committing it to vylog if the LSM tree has been dropped.
This should work just fine - since the LSM tee isn't used anymore we
don't need to have it compacted, neither do we need to delete the run,
since gc will eventually clean up all artefacts left from the dropped
LSM tree.

One thing to be noted is that we also must exclude dropped LSM trees
from further compaction - if we don't do that, we might end up picking
the dropped LSM tree for compaction over and over again (because it
isn't actually compacted).

This patch also drops the gh-5141-invalid-vylog-file test, because the
latter just ensured that the issue fixed by this patch is there.

Closes #5436