[stm] Remove unused "Proof using" data in `Sync` tags.
[stm] Remove unused "Proof using" data in `Sync` tags.

This was not used anywhere; it looks like dead code from some previous
attempt.

`get_hint_ctx` looks also very very suspicious.

ejgallego/coq,SkySkimmer/coq,ejgallego/coq,silene/coq,ppedrot/coq,maximedenes/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,JasonGross/coq,letouzey/coq-wip,maximedenes/coq,ppedrot/coq,gares/coq,Matafou/coq,letouzey/coq-wip,ejgallego/coq,Zimmi48/coq,silene/coq,SkySkimmer/coq,ppedrot/coq,Matafou/coq,letouzey/coq-wip,letouzey/coq-wip,coq/coq,JasonGross/coq,silene/coq,Zimmi48/coq,Zimmi48/coq,JasonGross/coq,maximedenes/coq,letouzey/coq-wip,Matafou/coq,coq/coq,coq/coq,maximedenes/coq,ppedrot/coq,maximedenes/coq,gares/coq,Zimmi48/coq,gares/coq,gares/coq,coq/coq,JasonGross/coq,ppedrot/coq,gares/coq