Fix help command in cogs

[plun1331]

Summary

This PR fixes the following exception:

Traceback (most recent call last):
  File "c:/Users/cplun/Documents/GitHub/pycord/test.py", line 9, in <module>
    bot.remove_cog('cog')
  File "c:\Users\cplun\Documents\GitHub\pycord\discord\cog.py", line 587, in remove_cog
    help_command = self._help_command
AttributeError: 'Bot' object has no attribute '_help_command'

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, ...)

[BobDotCom]

I think it may be added at some point

The base branch was changed.