compaction_strategy: delete major_compaction_strategy class
compaction_strategy: delete major_compaction_strategy class

It was already unused before this series. In an earlier version I have
used it to provide an ad-hoc backlog for major compactions. But now that
this is done by the compaction manager, this class really isn't being
used.

And it is likely it won't be: major compaction is not a compaction
strategy a user can choose, unlike the others that need to be built
through make_compaction_strategy.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

duarten/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla