Remove DiskQuota model

[hackdna]

Close #2465

[jkmarx]

Do we still need the DiskQuotaAdmin class?

[codecov]

Codecov Report

Merging #2599 into develop will decrease coverage by 2.77%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2599      +/-   ##
===========================================
- Coverage    55.76%   52.98%   -2.78%     
===========================================
  Files          408      406       -2     
  Lines        29588    26409    -3179     
  Branches      1240     1240              
===========================================
- Hits         16500    13994    -2506     
+ Misses       13088    12415     -673

Impacted Files	Coverage Δ
refinery/core/models.py	71.33% <ø> (-3.32%)	⬇️
refinery/core/admin.py	85.26% <100%> (-0.46%)	⬇️
refinery/analysis_manager/utils.py	55.1% <0%> (-9.98%)	⬇️
refinery/data_set_manager/tasks.py	36.99% <0%> (-5.52%)	⬇️
refinery/data_set_manager/views.py	45.49% <0%> (-5.01%)	⬇️
refinery/file_store/models.py	62.45% <0%> (-4.3%)	⬇️
refinery/analysis_manager/views.py	46.78% <0%> (-3.55%)	⬇️
refinery/core/views.py	26.57% <0%> (-3.15%)	⬇️
...ery/tool_manager/management/commands/load_tools.py	85.03% <0%> (-1.21%)	⬇️
... and 18 more

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[mccalluc]

Looks good.