configure: drop IRIX and AIX flags for --enable-optimize
configure: drop IRIX and AIX flags for --enable-optimize

IRIX is long dead (sorry), and AIX is not within our reach, so remove
outdated optimisation flags for them.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb