Revert "nir: const `nir_call_instr::callee`"

Revert "nir: const `nir_call_instr::callee`"

This reverts commit db57db5317e81fb4ce31bc294fdcc199db651542.  When
building IR, nothing is really immutable and, since C has no concept of
constness propagating beyond the first pointer, we have to be vary
careful with how we use it.  To just throw const into a function like
this is a lie.

Instead, we should just drop the unneeded const in spirv_to_nir which
this commit does along with the revert.