After commit a57724e711, off-strategy no longer races with view
building, therefore deletion code can be simplified and piggyback
on mechanism for deleting all sstables atomically, meaning a crash
midway won't result in some of the files coming back to life,
which leads to unnecessary work on restart.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#12245