fix(toastr): dispose no longer used overlay container

[yggg]

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• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Fixes memory leak when overlay container removed from body (for instance, when nb-layout changed) was just replaced by a new one without disposing previous.

[codecov]

Codecov Report

Merging #1873 into master will increase coverage by <.01%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master    #1873      +/-   ##
==========================================
+ Coverage   84.18%   84.19%   +<.01%     
==========================================
  Files         235      235              
  Lines        7899     7902       +3     
  Branches      691      692       +1     
==========================================
+ Hits         6650     6653       +3     
  Misses       1046     1046              
  Partials      203      203

Impacted Files	Coverage Δ
...ramework/theme/components/toastr/toastr.service.ts	90.67% <100%> (+0.16%)	⬆️
...me/components/toastr/toastr-container.component.ts	66.66% <40%> (+1.28%)	⬆️