Remove signed.v

[proux01]

The file is deprecated since 1.9.0

C.f. math-comp/math-comp#1584

Motivation for this change

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[proux01]

@affeldt-aist CI green, I let you decide what you want to do with this.