Add default value for target in prune cli

[sooahleex]

Summary

• Fix prune cli Raised AttributeError when run "datum prune" command without having additional argurment #1225
• Remove unnecessary line for dataset saving

How to test

• Use existed cli test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a819dbb) 80.57% compared to head (1521b49) 80.57%.

Files	Patch %	Lines
src/datumaro/cli/commands/prune.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1253   +/-   ##
========================================
  Coverage    80.57%   80.57%           
========================================
  Files          271      271           
  Lines        30441    30442    +1     
  Branches      5930     5930           
========================================
+ Hits         24528    24529    +1     
  Misses        4525     4525           
  Partials      1388     1388           

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	80.55% <33.33%> (+<0.01%)	⬆️
windows-2022_Python-3.8	80.54% <33.33%> (+<0.01%)	⬆️

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