Clear tracks when using "Return to import form"

[cmdcolin]

Reported by https://github.com/li153348734 in the gitter chat, this is a small fix where the tracks remained after using "Return to import form" and changing assemblies but the tracks weren't cleared out https://files.gitter.im/5c59cb65d73408ce4fb6f58e/BIz5/image.png

I checked other options in the linear-genome-view state that might also need to be "cleared" but didn't see any. Any further update that might need clearing can go into "clearView"

[codecov]

Codecov Report

Merging #1815 (cac3527) into master (e2594e3) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1815      +/-   ##
==========================================
- Coverage   58.85%   58.84%   -0.02%     
==========================================
  Files         459      459              
  Lines       21389    21390       +1     
  Branches     5051     5051              
==========================================
- Hits        12589    12587       -2     
- Misses       8489     8492       +3     
  Partials      311      311              

Impacted Files	Coverage Δ
...s/linear-genome-view/src/LinearGenomeView/index.ts	83.27% <0.00%> (-0.15%)	⬇️
plugins/alignments/src/BamAdapter/BamAdapter.ts	69.51% <0.00%> (-1.22%)	⬇️
...ments/src/SNPCoverageAdapter/SNPCoverageAdapter.ts	94.16% <0.00%> (-0.84%)	⬇️

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[rbuels]

The diff tab on Github shows a typecheck error coming from the typecheck github action, could you have a look