Don't special case ifxCheckForAfterRowTriggers() for version >= 9.3.

With commit 0eef1cef82b633082f4577a47c1df52abb17cd24 PostgreSQL 9.2 cannot
build the Informix FDW because we accidently special cased
ifxCheckForAfterRowTriggers() for version >= 9.3. Move the function
into the main code instead. This means that all supported versions will
compile with that function, even if not really needed. However, i don't
think that makes a big difference and i'd rather avoid introducing more
special cased code parts.