keep the RAID+AHCI setting, as this is still better than IDE mode which is primarily intended for legacy operating systems (XP and earlier) which cannot handle SATA drives.
If you cannot go without AHCI, you could get a PCIe S-ATA controller (preferably 3rd gen) and connect the SSD to the card.
As I wrote, the SSD is going to run fine when connected to the onboard S-ATA, but without advanced features due to lack of AHCI mode.
