Clean up implicit function imports

[Robbybp]

Summary/Motivation:

I noticed several unused imports in pyomo.contrib.pynumero.algorithms.solvers.implicit_functions, and one import that was causing a deprecation warning.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (67e900c) 87.46% compared to head (688668b) 87.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2892      +/-   ##
==========================================
- Coverage   87.46%   87.45%   -0.01%     
==========================================
  Files         771      771              
  Lines       89651    89646       -5     
==========================================
- Hits        78410    78398      -12     
- Misses      11241    11248       +7     

Flag	Coverage Δ
linux	84.52% <100.00%> (-0.01%)	⬇️
osx	74.57% <100.00%> (-0.02%)	⬇️
other	84.69% <100.00%> (-0.01%)	⬇️
win	82.10% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../pynumero/algorithms/solvers/implicit_functions.py	96.47% <100.00%> (-0.08%)	⬇️

... and 2 files with indirect coverage changes

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