Remove the wrapping function in SPIR-V (de)serialization

Remove the wrapping function in SPIR-V (de)serialization

Previously Module and Function are builtinn constructs in MLIR.
Due to the structural requirements we must wrap the SPIR-V
module inside a Function inside a Module. Now the requirement
is lifted and we can remove the wrapping function! :)

PiperOrigin-RevId: 264736051