lib/oid_registry.c: remove redundant assignment to variable num

lib/oid_registry.c: remove redundant assignment to variable num

The variable num is being assigned a value that is never read, it is being
re-assigned a new value in both paths if an if-statement.  The assignment
is redundant and can be removed.

Cleans up clang scan build warning:
lib/oid_registry.c:149:3: warning: Value stored to 'num' is
never read [deadcode.DeadStores]

Link: https://lkml.kernel.org/r/20221017214556.863357-1-colin.i.king@gmail.com
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>