Remove duplicate lines of code

Remove duplicate lines of code

Commit 6df7a9698bb accidentally included two identical prototypes for
default_multirange_selectivi() and commit 086cf1458c6 added a break;
statement where one was already present, thus duplicating it.  While
there is no bug caused by this, fix by removing the duplicated lines
as they provide no value.

Backpatch the fix for duplicate prototypes to v14 and the duplicate
break statement fix to all supported branches to avoid backpatching
hazards due to the removal.

Reported-by: Anton Voloshin 
Discussion: https://postgr.es/m/0e69cb60-0176-f6d0-7e15-6478b7d85724@postgrespro.ru