Update problem stats on submission delete

[Ninjaclasher]

No description provided.

[codecov-commenter]

Codecov Report

Merging #1438 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
- Coverage   38.43%   38.42%   -0.02%     
==========================================
  Files         201      201              
  Lines       11407    11410       +3     
==========================================
  Hits         4384     4384              
- Misses       7023     7026       +3     

Impacted Files	Coverage Δ
judge/signals.py	42.85% <0.00%> (-2.15%)	⬇️

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[Xyene]

Thanks!