nir/linker: remove reference to just SPIR-V linking

nir/linker: remove reference to just SPIR-V linking

Several files had a initial comment about the purpose of such files,
including a reference that the NIR linker was implemented with just
ARB_gl_spirv in mind.

Since the nice job Timothy is doing to use the NIR linker on GLSL,
that is not true anymore, so let's remove that reference and also
tweak some other comments.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4081>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4081>