Remove mistakenly added columns

[bpkroth]

#628 mistakenly included an early attempt at adding optimization_target and optimization_direction to the experiment table in the mlos_bench.storage.sql backend.

In that PR we later moved it to its own objectives table to eventually support multi-objectives.

Nothing accesses those columns now, however including them in the metadata makes it impossible to load storage backends previously created with the old schema since adjusting columns with sqlalchemy's create_all() API only considers table existence.

On the contrary, the latter means that we will automatically support old storage backends with the new code for the objectives table.

Removing these two columns in the metadata schema description simply allows that to proceed without error.

See Also: #649