duplicate function declaration in multirangetypes_selfuncs.c

Hello, hackers,

we have a duplicate line, declaration of default_multirange_selectivity() in
src/backend/utils/adt/multirangetypes_selfuncs.c:

static double default_multirange_selectivity(Oid operator);
static double default_multirange_selectivity(Oid operator);

Affected branches: REL_14_STABLE and above.

Both lines come from the same commit:
 > commit 6df7a9698bb036610c1e8c6d375e1be38cb26d5f
 > Author: Alexander Korotkov <akorotkov@postgresql.org>
 > Date:   Sun Dec 20 07:20:33 2020
 >
 >     Multirange datatypes

No harm from this duplication, still, I suggest to clean it up for 
tidiness' sake.

-- 
Anton Voloshin
Postgres Professional, The Russian Postgres Company
https://postgrespro.ru