Since these were reverse-engineered, it's become clear that IBO
descriptors are just a subset of texture descriptors, and bindless reads
of readonly images actually use isam on the IBO descriptor, further
confirming that the two are always compatible, even if not all of the
texture fields exist for IBOs. It's pointless to have a separate type
for IBOs, and just leads to things getting out-of-sync unnecessarily
which has already happened. Just remove it and use TEX_CONST insted.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15114>
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