Remove IfElse.jl

[devmotion]

This PR removes remaining traces of IfElse.jl.

Fixes #900.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 8.83%. Comparing base (eced900) to head (a2ba1fc).
Report is 30 commits behind head on master.

Files with missing lines	Patch %	Lines
src/extra_functions.jl	0.00%	5 Missing ⚠️
src/diff.jl	0.00%	1 Missing ⚠️
src/num.jl	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1449      +/-   ##
=========================================
- Coverage    8.88%   8.83%   -0.05%     
=========================================
  Files          51      51              
  Lines        4876    4903      +27     
=========================================
  Hits          433     433              
- Misses       4443    4470      +27     

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[ChrisRackauckas]

Wow, I completely forgot we had any IfElse.jl left 😅

[devmotion]

I just noticed that SymbolicUtils also still depends on it: JuliaSymbolics/SymbolicUtils.jl#706 😄