Remove SoftDelete from magma wrapper code

SoftDelete was added to support the removal of magma kvstores when prepareToDelete was called. This is no longer needed when CBSS-457 is checked in. But, to check in that fix, we first need to remove the code from kv engine magma code. 

CBSS-457 adds checking to kvstore creation and deletion so they become atomic operations.

kvstore revision is maintained by the magma kv engine code. When magma is instantiated, we get the current kvstore revision from magma. When prepareToCreate is called, we increment the kvstore revision. kvstore revision can be reset back to 1 when a kvstore is deleted, kv engine is shutdown and restarted. Since there is no active kvstore, the revision is reset to 1. Magma will remove old or invalid (partially created or deleted kvstores) revisions during magma recovery.