Revert "Split UBI_21 into UBI_2164 and UBI_65 [3.0]"

[MattHJensen]

Reverts #2463

cc @Peter-Metz, @MaxGhenis

[codecov]

Codecov Report

Merging #2471 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2471   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          13       13           
  Lines        2582     2582           
=======================================
  Hits         2580     2580           
  Misses          2        2           

Impacted Files	Coverage Δ
taxcalc/policy.py	100.00% <ø> (ø)
taxcalc/parameters.py	99.33% <100.00%> (ø)

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[Peter-Metz]

@MattHJensen the changes in the code base look good to me, but this is reverting some stuff we'll want to keep in the documentation (e.g. nu06 and parameter_indexing_CPI_offset)

cc @MaxGhenis

[MaxGhenis]

@MattHJensen the changes in the code base look good to me, but this is reverting some stuff we'll want to keep in the documentation (e.g. nu06 and parameter_indexing_CPI_offset)

Should be fine, I wasn't looking to include it anyway (just did it to test and didn't revert the commit). #2450 will ensure the next release reflects the documentation changes, including running make_uguide.py which produced the field changes.