utf8: Remove unused utf8_normalize

utf8_normalize() was added in 2019 as part of
commit 9d53690f0d4e ("unicode: implement higher level API for string
handling")
but has remained unused.

(I think because the other higher level routines added by that patch
normalise as part of their operations)

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>