Remove invalid requirement gsibec ~mkl from site configs

[climbfuji]

Description

Remove invalid requirement gsibec ~mkl from site configs since gsibec doesn't have this variant. It enables the use of MKL based on the virtual provider:

https://github.com/JCSDA/spack-packages/blob/fdb8a3c7ef02eb714afbd85dce57b388357514da/repos/spack_repo/builtin/packages/gsibec/package.py#L74

Dependencies

None

Issues addressed

Cleanup.

Applications affected

None

Systems affected

None

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped - site config updates only don't trigger CI builds
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • ...

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation (spack-stack wiki) will be made when this PR is merged

[climbfuji]

@mathomp4 If we do this first, then you have to make way fewer changes (only two places in configs/)