bug#31233 mysql_alter_table() fails to drop UNIQUE KEY

mysql-test/suite/ndb/r/ndb_alter_table.result:
  bug#31233 mysql_alter_table() fails to drop UNIQUE KEY: added test cases
mysql-test/suite/ndb/t/ndb_alter_table.test:
  bug#31233 mysql_alter_table() fails to drop UNIQUE KEY: added test cases
sql/ha_ndbcluster.cc:
  bug#31233 mysql_alter_table() fails to drop UNIQUE KEY: Removed check for non-pk
  tables, not needed when mysql_alter_table checks apropriate flags
sql/mysql_priv.h:
  bug #31231  mysql_alter_table() tries to drop a non-existing table: added FRM_ONLY
  flag
sql/sql_table.cc:
  bug #31231  mysql_alter_table() tries to drop a non-existing table
  Don't invoke handler for tables defined with FRM_ONLY flag.
  bug#31233 mysql_alter_table() fails to drop UNIQUE KEY
  When a table is defined without an explicit primary key
  mysql will choose the first found unique index defined over
  non-nullable fields (if such an index exists). This means
  that if such an index is added (the first) or dropped (the last)
  through an alter table, this equals adding or dropping a primary key.
  The implementation for on-line add/drop index did not consider
  this semantics. This patch ensures that only handlers with the
   correctly defined flags (see handler.h for explanation of the flags):
  HA_ONLINE_ADD_PK_INDEX
  HA_ONLINE_ADD_PK_INDEX_NO_WRITES
  HA_ONLINE_DROP_PK_INDEX
  HA_ONLINE_DROP_PK_INDEX_NO_WRITES
  are invoked for such on-line operations. All others handlers must
  perform a full (offline) alter table.