Remove strax.plugin

[dachengx]

What is the problem / what does the code in this PR do

Previously, strax.plugin.CutPlugin or from strax.plugin import CutPlugin will cause error:

>>> strax.plugin.CutPlugin
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'strax.plugins.plugin' has no attribute 'CutPlugin'
>>> from strax.plugin import CutPlugin
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/xudc/strax/strax/plugin.py", line 2, in <module>
    from plugins import (
ModuleNotFoundError: No module named 'plugins'

This means that strax/plugin.py is never properly used., so we delete them.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 91.538% (+0.03%) from 91.504%
when pulling 1e7205f on rm_plugin
into 4305e99 on master.