Remove epigraph formulation of quadratic objectives for ConicModel

[odow]

Closes https://github.com/lanl-ansi/PowerModels.jl/issues/55

We might need to test this on a much larger problem, since there are some test differences. I'll comment in-line on some things.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.73%. Comparing base (685fbb2) to head (b727e5d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #969      +/-   ##
==========================================
+ Coverage   93.71%   93.73%   +0.02%     
==========================================
  Files          43       43              
  Lines        9635     9602      -33     
==========================================
- Hits         9029     9000      -29     
+ Misses        606      602       -4     

Files with missing lines	Coverage Δ
src/core/objective.jl	96.37% <ø> (+1.63%)	:arrow_up:

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[ccoffrin]

I am thinking it is best to put this in the next breaking release, in an abundance of caution.