Remove the 'singletest' job from "branches" builds

[jsiirola]

Fixes # .

Summary/Motivation:

This should significantly decrease the load that the Pyomo GHA jobs put on the NEOS servers. It has no effect on PRs or coverage (the singletest job is still in test_pr_and_main.yml)

Changes proposed in this PR:

• remove the linux/singletest job from the branches GHA suite

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4f2d5fc) 87.82% compared to head (ccb3ffd) 87.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2917   +/-   ##
=======================================
  Coverage   87.82%   87.83%           
=======================================
  Files         770      770           
  Lines       89637    89637           
=======================================
+ Hits        78726    78734    +8     
+ Misses      10911    10903    -8     

Flag	Coverage Δ
linux	84.89% <ø> (ø)
osx	74.44% <ø> (-0.19%)	⬇️
other	85.07% <ø> (-0.01%)	⬇️
win	81.97% <ø> (-0.19%)	⬇️

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see 2 files with indirect coverage changes

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