refactor: remove ELAST across BSDs

[dybucc]

Description

This PR completely removes the symbols that were deprecated in #5118. This follows from the comments
by @JohnTitor in [1]. In this instance, removal implies both deletion in the souce file where the
constant is declared and purging of the corresponding symbols in the SemVer-tracking file.

Sources

See #5118.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

Some changes occurred in a NetBSD-like module

cc @semarie

Some changes occurred in an OpenBSD module

cc @semarie

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[dybucc]

CI actually passes. There seems to be an issue with a glob import that is not used, but this has not
been changed in the patch (it's not even part of it, for that matter.) For some reason, rebasing
onto main with dependabot updates has ended up with a warning across all of my open PRs due to
that one (now apparently unused) import.