remove never used param skip_list and fix doc string

[LeoJhonSong]

Motivation

It seems code of SimpleMPJPE() in mmpose/evaluation/metrics/simple_keypoint_3d_metrics.py was modified based on MPJPE() in mmpose/evaluation/metrics/keypoint_3d_metrics.py, however the parameter skip_list was never used in SimpleMPJPE() and should be removed to eliminate confusion. Besides, doc string about mode of SimpleMPJPE() and MPJPE() has mistake.

Modification

• unused code and doc string related to skip_list are removed
• fix doc string of MPJPE() and SimpleMPJPE()

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.

[CLAassistant]

All committers have signed the CLA.