pgsql: Silence warning in non-assert-enabled build

Silence warning in non-assert-enabled build

An OID return value was being used only for a (rather pointless) assert.
Silence by removing the variable and the assert.

Per note from Peter Geoghegan

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/bf22d2707a2f47a7cc4caa239a14f2bf0a72bfd0

Modified Files
--------------
src/backend/catalog/pg_conversion.c |    4 +---
1 file changed, 1 insertion(+), 3 deletions(-)