remove quantile QTL code and other cleanup

[danielnachun]

This PR fixes is mostly just a massive cleanup:

• removes all quantile QTL code which should now live in the qQTLR repository
• removes the plotting and UDR code
• replaces UDR in the mash wrapper with the standard mashr covariance functions
• optimizes and cleans up code in various places where we now have enough test coverage to make those changes confidently

The only user-facing changes are the deletion of the quantile QTL functions.

[codecov]

Codecov Report

❌ Patch coverage is 94.62151% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.80%. Comparing base (0517999) to head (8e75d87).
⚠️ Report is 1037 commits behind head on main.

Files with missing lines	Patch %	Lines
R/mash_wrapper.R	84.90%	16 Missing ⚠️
R/mr.R	94.50%	5 Missing ⚠️
R/colocboost_pipeline.R	94.11%	4 Missing ⚠️
R/twas.R	98.87%	1 Missing ⚠️
R/twas_weights.R	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #471       +/-   ##
===========================================
+ Coverage   27.48%   83.80%   +56.32%     
===========================================
  Files          32       35        +3     
  Lines        4377     6527     +2150     
===========================================
+ Hits         1203     5470     +4267     
+ Misses       3174     1057     -2117     

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