company-coq-lemma-from-goal should be adapted w.r.t. Coq Diffs

[erikmd]

When the DIffs feature (recently added in PG by @jfehrle) is enabled (menu Coq > Diffs > "on"),
the extraction lemma C-c C-a C-x doesn't work out-of-the-box.

MWE:

Require Import Bool.
Lemma test : forall a b : bool, a && b = b && a.
destruct a; simpl; intros b.

• Do C-c C-RET then C-c C-a C-x foo RET
• Then we get:

Lemma foo:
  <diff.added.bg><diff.added>forall b : bool,</diff.added> b = b && true</diff.added.bg>.
                                    Proof.
                                    intros b.

so the <diff.*> pseudo-tags should be removed.