tr_shade: do not use reflective specular code on PBR material

[illwieckz]

It is not implemented yet and the shader permutation is disabled.

It now makes possible to run buildcubemaps command even when there are PBR materials in the scene, to not crash while attempting to render those PBR materials with a shader permutation that doesn't exist.

To test the reflective specular code, one should:

• load a map with specular maps
• have r_smp disabled (there are some crashes when generating cubemaps otherwise)
• type /buildcubemaps in console

The rendering is assumed to be wrong (it blinks to black), but the buildcubemaps command runs.