Revert "[MLIR][SPIRV] Start module combiner"

This reverts commit 316593ce.
Build is broken with:

TestModuleCombiner.cpp:(.text._ZN12_GLOBAL__N_122TestModuleCombinerPass14runOnOperationEv+0x195): undefined reference to `mlir::spirv::combine(llvm::MutableArrayRef<mlir::spirv::ModuleOp>, mlir::OpBuilder&, llvm::function_ref<void (mlir::spirv::ModuleOp, llvm::StringRef, llvm::StringRef)>)'