Remove TreeBuilder and all the dead code associated from PBT

[h3xds1nz]

Description

Removes TreeBuilder, which is just nowadays an unused abstract class from PBT and the associated dead code.
This was only in NetFX 3.5, it is unused since NetFX 4.0.

Customer Impact

Cleaner codebase for developers.

Regression

No.

Testing

Local build.

Risk

None, unused dead code.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.07186%. Comparing base (6514196) to head (a80df09).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10635         +/-   ##
===================================================
- Coverage   11.22814%   11.07186%   -0.15629%     
===================================================
  Files           3352        3352                 
  Lines         668000      668000                 
  Branches       74980       74980                 
===================================================
- Hits           75004       73960       -1044     
- Misses        591745      592881       +1136     
+ Partials        1251        1159         -92     

Flag	Coverage Δ
Debug	11.07186% <ø> (-0.15629%)	⬇️

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