kbuild: remove LLVM=1 test from HAS_LTO_CLANG

As Documentation/kbuild/llvm.rst notes, LLVM=1 switches the default of
tools, but you can still override CC, LD, etc. individually. This LLVM=1
check is unneeded because each tool is already checked separately.

"make CC=clang LD=ld.lld NM=llvm-nm AR=llvm-ar LLVM_IAS=1 menuconfig"
should be able to enable Clang LTO.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>