You can't do that. Once a Steam account is linked with the Social Club it's permanently linked and you have to contact support.
I have the exact same issue. I played the game on PS3 with a Social Club account and didn't realise I had a totally different one associated with Steam from ages ago. Now I can't play the game until Rockstar support removes my Steam account from that Social Club profile.
I submitted my support ticket 14 hours ago and there's been no response. They replied to one of my four tweets with a generic 'we've found your ticket and will look into it shortly'. That was five hours ago.