Remove code that was required for astropy<4

[astrofrog]

Also updated two reference datasets to include those keywords

[codecov]

Codecov Report

Merging #346 (8e2c39b) into main (bdadcb7) will decrease coverage by 2.62%.
The diff coverage is n/a.

❗ Current head 8e2c39b differs from pull request most recent head 9a54ddf. Consider uploading reports for the commit 9a54ddf to get more accurate results

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
- Coverage   92.39%   89.77%   -2.62%     
==========================================
  Files          24       24              
  Lines         802      802              
==========================================
- Hits          741      720      -21     
- Misses         61       82      +21     

Impacted Files	Coverage Δ
reproject/mosaicking/wcs_helpers.py	86.02% <0.00%> (-13.98%)	⬇️
reproject/wcs_utils.py	83.33% <0.00%> (-12.50%)	⬇️
reproject/mosaicking/coadd.py	84.84% <0.00%> (-7.58%)	⬇️

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[astrofrog]

I need to update more reference files