Remove `Berny` optimizer

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.

See this comment for justification.

Changes:

• Remove ase.optimize.berny and related tests
• Remove ase.optimize.optimize.OptimizableAtoms.get_chemical_symbols
  • AFAICT, this was only necessary for the Berny optimizer

Note: pipelines are currently failing due to minutes