x86/mm/64: Make 5-level paging support unconditional

Both Intel and AMD CPUs support 5-level paging, which is expected to
become more widely adopted in the future. All major x86 Linux
distributions have the feature enabled.

Remove CONFIG_X86_5LEVEL and related #ifdeffery for it to make it more readable.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250516123306.3812286-4-kirill.shutemov@linux.intel.com