mysql: remove obsolete comment about broken PARTITIONing support

Using NEO 0.9.1 and partitioning enabled, I could reproduce the issue with
MariaDB 5.3.5, but not with MariaDB 5.3.12 and 5.5.23. I suppose it was fixed.

In testOudatedCellsOnDownStorage,
  'select count(*) from obj' returned a wrong value (always 1).

Strangely, 'select count(*) from test_neo0.obj' was always correct (102).

neo/storage/database/mysqldb.py

@@ -53,9 +53,6 @@ class MySQLDatabaseManager(DatabaseManager):
     ENGINES = "InnoDB", "RocksDB", "TokuDB"
     _engine = ENGINES[0] # default engine
 
-    # Disabled even on MySQL 5.1-5.5 and MariaDB 5.2-5.3 because
-    # 'select count(*) from obj' sometimes returns incorrect values
-    # (tested with testOudatedCellsOnDownStorage).
     _use_partition = False
 
     _max_allowed_packet = 32769 * 1024