Preserve the dimensions of ReverseDiff TrackedArray

[avik-pal]

• Fixes reversediff flattening arrays
• Removes old code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.16%. Comparing base (ae06bba) to head (f3f07b6).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   70.56%   71.16%   +0.60%     
==========================================
  Files           7        8       +1     
  Lines         547      548       +1     
==========================================
+ Hits          386      390       +4     
+ Misses        161      158       -3     

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[avik-pal]

@ChrisRackauckas I am guessing the sign difference is coming from the diffeqcallbacks bug that you fixed and that hasn't been propagated to GaussAdjoint?