Remove unnecessary global variable and unit tests.

[mairanteodoro]

Closes #1306

This PR eliminates the global variable ALIGN_TO_ABS_REFCAT from TweakRegStep and the two unit tests that used it. Since TweakRegStep is always used for absolute astrometry, there is no need to have that global variable with fixed value.

Regression Tests
All tests are passing:

• https://github.com/spacetelescope/RegressionTests/actions/runs/9983530132

Checklist

• added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.28%. Comparing base (af07aa2) to head (344a1c2).
Report is 248 commits behind head on main.

Files with missing lines	Patch %	Lines
romancal/tweakreg/tweakreg_step.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1314      +/-   ##
==========================================
- Coverage   79.37%   79.28%   -0.10%     
==========================================
  Files         117      117              
  Lines        8086     8065      -21     
==========================================
- Hits         6418     6394      -24     
- Misses       1668     1671       +3     

Flag	Coverage Δ		*Carryforward flag
nightly	62.98% <ø> (+0.11%)	⬆️	Carriedforward from af07aa2

*This pull request uses carry forward flags. Click here to find out more.

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[ddavis-stsci]

LGTM