nv50/ir: remove symbol table support for compute shaders

nv50/ir: remove symbol table support for compute shaders

The initial plan was to use this for OpenCL kernels, but back then the
plan was to convert from LLVM to TGSI. As it turns out, we didn't went
that way.

Right now for OpenCL we don't reqiure supporting multiple entry points
inside the same binary and if we want to support it later, we can add
this back.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4264>