glsl: remove dead uniforms in nir
This is possible since we now do uniform linking via a nir based linker.
Shader-db results for IRIS (SKL):
total instructions in shared programs: 14947192 -> 14946397 (<.01%)
instructions in affected programs: 39498 -> 38703 (-2.01%)
helped: 230
HURT: 18
total cycles in shared programs: 324868402 -> 324847058 (<.01%)
cycles in affected programs: 706701 -> 685357 (-3.02%)
helped: 599
HURT: 449
This series causes some CTS test failures but it turns out that it exposed some CTS bugs. All CTS bugs now have there fixes merged upstream.