Make reflexivity hints work for evars.

Since Coq 8.4 did not backtrack on eauto premises, we used to ensure
that hints like

  Hint Extern 0 (?x ≡{_}≡ ?y) => reflexivity.

were not used for goals involving evars by writing ?x ≡{_}≡ ?y instead
of _ ≡{_}≡ _.

This seems to be a legacy issue that no longer applies to Coq 8.5, so
I have removed these restrictions making these hints thus more powerful.