intel/brw: Remove code for Gfx8-

On top of !27563 (merged), !27638 (merged), !27768 (merged) and !27766 (closed).

This removes dead code from BRW since Gfx8- is not used there anymore. Shouldn't affect functionality or existing shader code produced by the drivers.

The series of smaller commits should help reviewing, and bisecting future regressions.

Flattening of the now unnecessary class hierarchies involving fs / backend types will be handled in a separate MR.