linker.cpp: remove workaround for an old bug in LLVM
linker.cpp: remove workaround for an old bug in LLVM

There was an old workaround to fix image-type address spaces
in the linker because the llvm::CloneFunction didn't clone
AS correctly. This should be already fixed in all LLVM versions
we care about. For SPIR, AS of image types is now handled properly
by the SPIR wrappers.

pocl/pocl,pocl/pocl,pocl/pocl,pocl/pocl,pocl/pocl