runtime: drop unused exit()

[arajasek]

I love negative lines of code.

[codecov-commenter]

Codecov Report

Merging #1162 (5ec2d30) into next (6540db3) will increase coverage by 0.03%.
The diff coverage is 66.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1162      +/-   ##
==========================================
+ Coverage   81.81%   81.84%   +0.03%     
==========================================
  Files         131      131              
  Lines       26559    26572      +13     
==========================================
+ Hits        21728    21749      +21     
+ Misses       4831     4823       -8     

Impacted Files	Coverage Δ
runtime/src/runtime/mod.rs	48.97% <ø> (ø)
runtime/src/test_utils.rs	63.74% <0.00%> (+0.47%)	⬆️
test_vm/src/lib.rs	62.40% <100.00%> (-0.85%)	⬇️
actors/power/src/lib.rs	83.58% <0.00%> (+0.21%)	⬆️
state/src/check.rs	86.35% <0.00%> (+0.26%)	⬆️
...ors/evm/src/interpreter/instructions/arithmetic.rs	92.18% <0.00%> (+0.52%)	⬆️
actors/miner/src/deadline_info.rs	96.62% <0.00%> (+16.85%)	⬆️

[vyzo]

dunno, we might want to use it one day...

[Stebalien]

@vyzo right now, the priority right now is merging to master and this code is unused and untested. If we do need it, we can always add it back (asr/runtime-exit)