nir/spirv: Remove the vtn_type argument from _vtn_variable_load/store
nir/spirv: Remove the vtn_type argument from _vtn_variable_load/store

Now that builtins are handled in deref chains, we don't really need this
anymore.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler