Revert ECPG's use of pnstrdup()

Revert ECPG's use of pnstrdup()

Commit 0b9466fce added a dependency on fe_memutils' pnstrdup() inside
informix.c.  This adds an exit() path in a library, which we don't
want.  (Unlike libpq, the ecpg libraries don't have an automated check
for that, but it makes sense to keep them to a similar standard.)  The
ecpg code can already handle failure results from the *strdup() call
by itself.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAOYmi+=pg=W5L1h=3MEP_EB24jaBu2FyATrLXqQHGe7cpuvwyg@mail.gmail.com