Remove unnecessary actor system termination in example apps

[jchapuis]

It's already handled by AkkaCluster/PekkoCluster managed resources

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa1ba0f) 89.73% compared to head (4af1ebf) 89.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
- Coverage   89.73%   89.61%   -0.12%     
==========================================
  Files          73       73              
  Lines         877      867      -10     
  Branches       94       98       +4     
==========================================
- Hits          787      777      -10     
  Misses         90       90              

Files	Coverage Δ
.../main/scala/endless/example/app/akka/AkkaApp.scala	100.00% <100.00%> (ø)
...ain/scala/endless/example/app/pekko/PekkoApp.scala	100.00% <100.00%> (ø)
...la/endless/runtime/pekko/deploy/PekkoCluster.scala	92.30% <ø> (ø)