Remove useless asserts in report_namespace_conflict()

Remove useless asserts in report_namespace_conflict()

An assertion is used in this routine to check that a valid namespace OID
is given by the caller, but it was repeated twice: once at the top of
the routine and a second time multiple times in a switch/case.  This
commit removes the assertions within the switch/case.

Thinko in commit 765cbfdc9263.

Author: Man Zeng <zengman@halodbtech.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/tencent_40F8C1D82E2EE28065009AAA@qq.com