Remove useless --db-drop-deleted-data option from backend run command

[touilleMan]

This option has never done anything and it wasn't part of the documentation so I guess it's safe to remove it entirely for the moment ;-)

[codecov]

Codecov Report

Merging #1246 into master will increase coverage by 5.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1246      +/-   ##
==========================================
+ Coverage   69.87%   75.17%   +5.29%     
==========================================
  Files         298      298              
  Lines       26293    26305      +12     
==========================================
+ Hits        18372    19774    +1402     
+ Misses       7921     6531    -1390     

Impacted Files	Coverage Δ
parsec/backend/cli/run.py	70.62% <ø> (-0.19%)	⬇️
parsec/backend/config.py	93.44% <ø> (-0.11%)	⬇️
parsec/core/gui/_resources_rc.py	80.00% <0.00%> (-1.25%)	⬇️
parsec/core/fs/userfs/userfs.py	84.15% <0.00%> (+0.21%)	⬆️
parsec/core/gui/workspaces_widget.py	70.25% <0.00%> (+0.25%)	⬆️
parsec/core/gui/files_widget.py	76.47% <0.00%> (+0.53%)	⬆️
parsec/core/fs/workspacefs/entry_transactions.py	97.05% <0.00%> (+0.58%)	⬆️
parsec/core/types/backend_address.py	92.92% <0.00%> (+0.58%)	⬆️
parsec/backend/app.py	93.84% <0.00%> (+0.76%)	⬆️
parsec/core/gui/trio_thread.py	91.66% <0.00%> (+1.11%)	⬆️
... and 55 more

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