Removes unused code related to DatabaseTasks.

I found some unused code related to DatabaseTasks.

When I removed Oracle / SqlServer / Firebird Tasks, I also should have removed this code.

Related to https://github.com/rails/rails/commit/42d0d1832f3354dbbd61163c2bb9a7a50227d98b , https://github.com/rails/rails/commit/6b16b2f0f3f3a6800640024f729b862c05d1aa2d , https://github.com/rails/rails/commit/6649140120a5a6456d7fc63841d90d19a78b3b8c