riscv: Unconditionally use linker relaxation

Now that the minimum supported version of LLVM for building the kernel
has been bumped to 15.0.0, CONFIG_RISCV_USE_LINKER_RELAXATION will
always be enabled, so it can be removed.

Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250821-bump-min-llvm-ver-15-v2-8-635f3294e5f0@kernel.org


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