vinyl: fix gc of incomplete runs left from dropped indexes

If an index is dropped while a dump/compaction task is in progress, the
task will be aborted, but VY_LOG_DROP_RUN record won't be written to the
metadata log so the run will remain incomplete (VY_LOG_PREPARE_RUN) and
pin the dropped index in the log until the server is restarted and all
incomplete runs are purged. This results in occasional failures of the
vinyl/gc test which checks that vylog files are removed after all vinyl
spaces are dropped. There's actually no reason not to log deletion of an
incomplete run that belongs to a dropped index so let's do this to fix
this problem.

Closes #2486