Remove OBBBA folder

[martinholmer]

The OBBBA folder contained the information used to implement the OBBBA reform into current-law policy in the policy_current_law.json file. Now that that has been done, the contents of the OBBBA folder are obsolete, and therefore, being removed.

The wording of the taxcalc/reforms/OBBBA.md file has been changed to reflect the removal of the OBBBA folder.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d52cd4e) to head (ca32b5c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2986   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         2719      2719           
=========================================
  Hits          2719      2719           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.