Remove old datestring2ctime script

[BrianJKoopman]

Description

This PR removes the datestring2ctime script and supporting rename.py module. This was used very early on to mass rename output .g3 files from the aggregator when we transitioned to using the ctime based naming convention.

Motivation and Context

This continues some code clean up effort. The old datestring based format hasn't been used in at least five years, it should be safe to remove this by now -- anyone who has old style files either has renamed them by now, or doesn't care to.

How Has This Been Tested?

Not yet tested. CI will run tests, though I don't expect any impact besides decreased code coverage, since we're removing tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code cleanup

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.