`safe_foreach_sstable` doesn't do its job correctly.
It iterates over an sstable set under the sstable deletion
lock in an attempt to ensure that SSTables aren't deleted during the iteration.
The thing is, it takes the deletion lock after the SSTable set is
already obtained, so SSTables might get unlinked *before* we take the lock.
Remove this function and fix its usages to obtain the set and iterate
over it under the lock.
Closesscylladb/scylladb#23397
e23fdc0799