She doesn't need Pycelle.
Pycelle was the only potential dissenting voice. Naturally he couldn't do anything about CerceiCersei being in power and it's unlikely that he would have gone to much effort to undermine her. But CerceiCersei doesn't need him. And Pycelle doesn't like Qyburn, andwhom neither she nor Qyburn are fond of him.
Most of all, CerceiCersei is now CerceiCersei without her children, so she has absolutely nothing holding back her ruthless cruelty. Since she has Qyburn who is far more loyal and helpful, why not remove Pycelle?