lib/oid_registry.c: remove unused sprint_OID

lib/oid_registry.c: remove unused sprint_OID

sprint_OID() was added as part of 2012's commit 4f73175d0375 ("X.509: Add
utility functions to render OIDs as strings") but it hasn't been used.
Remove it.

Note that there's also 'sprint_oid' (lower case) which is used in a lot of
places; that's left as is except for fixing its case in a comment.

Link: https://lkml.kernel.org/r/20250501010502.326472-1-linux@treblig.org
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>