Remove OA_RETINTEGER, unused since 2002 (commit e7311069df54baa6)

Remove OA_RETINTEGER, unused since 2002 (commit e7311069df54baa6)

This returns us to 8 flag bits, and restores OCSHIFT and OASHIFT to 8 and 12
Previously these were 9 and 13, and effectively PL_opargs[] was using 33 of
32 bits, relying on the ugly hack that no 5 argument builtin had an optional
5th argument, hence the (13 + 5 * 4)th bit was always zero.
is effectively 33 bits.