Remove partial support for migration from tahoe-lafs magic-folder.

[tomprince]

Fixes #401.

[codecov]

Codecov Report

Merging #402 (897a375) into main (31531e4) will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
+ Coverage   85.43%   85.61%   +0.18%     
==========================================
  Files          35       34       -1     
  Lines        2753     2712      -41     
  Branches      317      308       -9     
==========================================
- Hits         2352     2322      -30     
+ Misses        321      315       -6     
+ Partials       80       75       -5     

Flag	Coverage Δ
integration	44.24% <ø> (-0.15%)	⬇️

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Impacted Files	Coverage Δ
src/magic_folder/cli.py	73.13% <ø> (+2.56%)	⬆️
src/magic_folder/config.py	96.16% <0.00%> (-1.10%)	⬇️

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[meejah]

I don't understand the motivation for removing a feature that works; see #401 for that discussion though.