lib/classifier: Simpilify array ordering.

The terminology we used for subtable ordering ('splice', 'right
before') was inherited from an earlier use of a linked list, and
turned out to be confusing when applied to an array.  Also, we only
ever move one subtable earlier or later within the array, so we can
simplify the code as well.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>