Remove test_obs directory - not used (built or run)

[hkershaw-brown]

Description:

code not used
Looks like the original code was/intended for creating data for obs_diag.
Discovered when I was updating the build system. Conservatively kept it in at that time. The code has not been in use for some time, so removing it rather than having dead code (not built, not run) around.

Fixes issue

fixes #220

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)
• Documentation update
• dead code removal

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Compiled everything in DART, grepped around for anything that was including the code.

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[mjs2369]

Clean removal