[InviteTracker] Fix.

[Honkertonken]

Hey there, made quite a lot of changes in invitetracker the cog should be working as expected right now.
Changes include -

1. Dropped the on_member_remove listener as it was not accurate and getting the invite on leave was going to be messy/ not accurate you can read more abut it here.
2. As leave tracking was not a thing anymore removed invset leaveenable.
3. Merged invset joinenable and invset enable into 1 command.
4. Fixed the userinvites command.
5. Some minor changes which includes using f strings, removed unused stuff in the config, grammar fixes etc (not very prominent).

Ps: Haven't updated the cog readme / version.

Do let me know if you have any suggestions.
Thanks in advance.