dvc: drop cached external outputs

[efiop]

Fixes #9531

Docs iterative/dvc.org#4574 Kudos @dberenbaum

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.48 ⚠️

Comparison is base (262419e) 90.73% compared to head (cf233e0) 90.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9570      +/-   ##
==========================================
- Coverage   90.73%   90.25%   -0.48%     
==========================================
  Files         470      470              
  Lines       36102    35980     -122     
  Branches     5207     5176      -31     
==========================================
- Hits        32757    32475     -282     
- Misses       2749     2869     +120     
- Partials      596      636      +40     

Impacted Files	Coverage Δ
dvc/cachemgr.py	90.16% <ø> (-6.76%)	⬇️
dvc/commands/add.py	87.27% <ø> (+2.78%)	⬆️
dvc/config_schema.py	89.09% <ø> (-3.64%)	⬇️
dvc/exceptions.py	93.79% <ø> (-0.13%)	⬇️
dvc/repo/add.py	97.14% <ø> (ø)
dvc/testing/workspace_tests.py	47.29% <ø> (-4.70%)	⬇️
tests/func/test_gc.py	100.00% <ø> (ø)
tests/func/test_get.py	100.00% <ø> (ø)
tests/func/test_odb.py	100.00% <ø> (ø)
tests/unit/command/test_add.py	100.00% <ø> (ø)
... and 10 more

... and 42 files with indirect coverage changes

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