[X86] Remove unused function (NFC)

llvm-project/llvm/lib/Target/X86/X86ISelLowering.cpp:3537:13:
error: unused function 'isAnyZeroOrUndef' [-Werror,-Wunused-function]
static bool isAnyZeroOrUndef(ArrayRef<int> Mask) {
            ^
1 error generated.