fix: remove explorer stuff

[fdrgsp]

This PR removes unused old explorer widget stuff.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.06% 🎉

Comparison is base (a1e60c6) 75.24% compared to head (746d4bd) 79.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #273      +/-   ##
==========================================
+ Coverage   75.24%   79.31%   +4.06%     
==========================================
  Files          15       14       -1     
  Lines         820      754      -66     
==========================================
- Hits          617      598      -19     
+ Misses        203      156      -47     

Files Changed	Coverage Δ
src/napari_micromanager/_saving.py	64.58% <ø> (+9.58%)	⬆️
src/napari_micromanager/_mda_handler.py	97.80% <100.00%> (+25.41%)	⬆️
src/napari_micromanager/_mda_meta.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[tlambert03]

always love to see it! :)