Revert "nir: const nir_call_instr::callee"

This reverts commit db57db5317.  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.

src/compiler/nir/nir.c

@@ -515,7 +515,7 @@ nir_intrinsic_instr_create(nir_shader *shader, nir_intrinsic_op op)
 }
 
 nir_call_instr *
-nir_call_instr_create(nir_shader *shader, const nir_function *callee)
+nir_call_instr_create(nir_shader *shader, nir_function *callee)
 {
    const unsigned num_params = callee->num_params;
    nir_call_instr *instr =