Documentation/llvm: drop note about LLVM=0

Documentation/llvm: drop note about LLVM=0

Since commit 502678b88cb3 ("kbuild: Reject unexpected values for LLVM=")
LLVM=0 generates an error instead of silently behaving unexpectedly.

Drop the now unnecessary note.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260226-kbuild-llvm-followup-v1-1-201cc2a492d9@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>