Remove useless pstrdups in untransformRelOptions

Remove useless pstrdups in untransformRelOptions

The two strings are already a single palloc'd chunk, not freed; there's
no reason to allocate separate copies that have the same lifetime.

This code is only called in short-lived memory contexts (except in some
cases in TopTransactionContext, which is still short-lived enough not to
really matter), and typically only for short arrays, so the memory or
computation saved is likely negligible.  However, let's fix it to avoid
leaving a bad example of code to copy.  This is the only place I could
find where we're doing this with makeDefElem().

Reported-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/20220909142050.3vv2hjekppk265dd@alvherre.pgsql