Refactored ha_ndbcluster::prepare_inplace_alter_table() and
ha_ndbcluster::prepare_drop_index()

changes :

ha_ndbcluster::prepare_inplace_alter_table()
 - index_drop_buffer is processed in a loop and at each run, and
   prepare_drop_index() is called for each index rather than being called
   at the end
 - This eliminates the need for storing index numbers. So the
   key_numbers is removed.

ha_ndbcluster::prepare_drop_index()
 - Now takes in only one argument - key_num and mark that as
   TO_BE_DROPPED in the m_index array.
 - Removed the other arguments

Also removed the unused implementation of ha_ndbcluster::add_index()