Silently ignore requests to _not_ clear something when that something cannot be cleared.
Silently ignore requests to _not_ clear something when that something cannot be cleared.

This should fix the contrib failures on tactics like "destruct (0)".

maximedenes/coq,Ptival/coq,amintimany/coq,coq/coq,maximedenes/coq,SkySkimmer/coq,mattam82/coq,Zimmi48/coq,amintimany/coq,william-lawvere/coq,silene/coq,letouzey/coq-wip,herbelin/github-coq,SkySkimmer/coq,Ptival/coq,letouzey/coq-wip,ejgallego/coq,herbelin/github-coq,JasonGross/coq,william-lawvere/coq,SkySkimmer/coq,JasonGross/coq,ppedrot/coq,Matafou/coq,Zimmi48/coq,mattam82/coq,Lionel-Rieg/coq,SkySkimmer/coq,ppedrot/coq,gares/coq,ppedrot/coq,gares/coq,maximedenes/coq,Zimmi48/coq,Matafou/coq,letouzey/coq-wip,Matafou/coq,ejgallego/coq,ppedrot/coq,JasonGross/coq,ppedrot/coq,maximedenes/coq,silene/coq,silene/coq,Ptival/coq,william-lawvere/coq,Lionel-Rieg/coq,letouzey/coq-wip,letouzey/coq-wip,gares/coq,mattam82/coq,ejgallego/coq,Lionel-Rieg/coq,herbelin/github-coq,maximedenes/coq,gares/coq,gares/coq,coq/coq,amintimany/coq,Zimmi48/coq,JasonGross/coq,coq/coq,ejgallego/coq,coq/coq