riscv: Remove version check for LTO_CLANG selects

Now that the minimum supported version of LLVM for building the kernel
has been bumped to 15.0.0, there is no need to check the LLD version
before selecting ARCH_SUPPORTS_LTO_CLANG{,_THIN} because it will always
be true.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250821-bump-min-llvm-ver-15-v2-7-635f3294e5f0@kernel.org


Signed-off-by: Nathan Chancellor <nathan@kernel.org>