x86/Kconfig: Clean up LLVM version checks in IBT configurations

The minimum supported version of LLVM for building the x86 kernel
was bumped to 15.0.0 in

  7861640a ("x86/build: Raise the minimum LLVM version to 15.0.0"),

so the checks for Clang 14.0.0 and ld.lld 14.0.0 or newer will always been
true. Clean them up.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250814-x86-min-ver-cleanups-v1-6-ff7f19457523@kernel.org