riscv: Drop tautological condition from TOOLCHAIN_NEEDS_OLD_ISA_SPEC

riscv: Drop tautological condition from TOOLCHAIN_NEEDS_OLD_ISA_SPEC

Now that the minimum supported version of LLVM for building the kernel
has been raised to 17.0.1, the Clang dependency part of
CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC is always false, so it can be
removed. Adjust the help text to remove mention of Clang < 17, as it is
irrelevant for the kernel after the minimum supported bump.

Acked-by: Paul Walmsley <pjw@kernel.org>  # arch/riscv
Link: https://patch.msgid.link/20260517-bump-minimum-supported-llvm-version-to-17-v2-9-b3b8cda46bdd@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>