mesa/st: Remove GL_ARB_depth_clamp emulation support.

This was useful for emulating GL 3.2 in virgl on a GLES3 host renderer, before GL_EXT_depth_clamp introduced the ability for hardware drivers to expose the feature on GLES. Now that we have that, the desktop-GL-capable HW that virgl cares about can expose desktop GL even on its GLES renderer on the host without this emulation. I don't think anyone particularly cares about hitting higher GL versions on actually-core-GLES hosts with virgl.