[Refactor] Remove webcam API

[Ben-Louis]

Motivation

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.46 🎉

Comparison is base (c6424b8) 80.70% compared to head (6f4b048) 81.17%.

❗ Current head 6f4b048 differs from pull request most recent head 7208da2. Consider uploading reports for the commit 7208da2 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2511      +/-   ##
===========================================
+ Coverage    80.70%   81.17%   +0.46%     
===========================================
  Files          251      226      -25     
  Lines        15218    13963    -1255     
  Branches      2636     2415     -221     
===========================================
- Hits         12282    11334     -948     
+ Misses        2273     2011     -262     
+ Partials       663      618      -45     

Flag	Coverage Δ
unittests	81.17% <100.00%> (+0.46%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmpose/version.py	47.05% <100.00%> (ø)

... and 5 files with indirect coverage changes

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