prune: Remove invalid files

[fd0]

Closes #1029

[codecov-io]

Codecov Report

Merging #1036 into master will decrease coverage by 0.08%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
- Coverage   54.03%   53.94%   -0.09%     
==========================================
  Files         114      114              
  Lines        8776     8785       +9     
==========================================
- Hits         4742     4739       -3     
- Misses       3349     3358       +9     
- Partials      685      688       +3

Impacted Files	Coverage Δ
src/cmds/restic/cmd_rebuild_index.go	68.29% <100%> (ø)	⬆️
src/restic/index/index.go	83.45% <33.33%> (-2.59%)	⬇️
src/cmds/restic/cmd_prune.go	61.74% <33.33%> (-1.45%)	⬇️
src/restic/backend/test/tests.go	56.71% <0%> (-0.9%)	⬇️
src/restic/archiver/archiver.go	64.52% <0%> (-0.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eadf5dc...1baaa77. Read the comment docs.