Clean up some leftover utils from legacy generators

[saitcakmak]

Summary:
Removes a bunch of model fitting & acqf construction utils that were used in legacy generators. These are not used anywhere since the legacy generator has been removed.

Also removed botorch_moo_defaults to botorch_moo_utils to better communicate its purpose.

Reviewed By: dme65

Differential Revision: D84653561

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D84653561.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.01961% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.21%. Comparing base (4f4071f) to head (48841de).

Files with missing lines	Patch %	Lines
ax/generators/torch/botorch_moo_utils.py	98.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4430      +/-   ##
==========================================
+ Coverage   96.16%   96.21%   +0.04%     
==========================================
  Files         555      553       -2     
  Lines       57269    56745     -524     
==========================================
- Hits        55071    54595     -476     
+ Misses       2198     2150      -48     

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