Fix: Action Items corrections & Cleanup

[meetulr]

Describe the bug

Some checks and cleanups are required in the code associated with action items and action item categories:

1. In createActionItem mutation, a check should be added for disabled action item category, i.e. we should not be able to create an action item for a disabled action item category.
2. In updateActionItemCategory mutation, an error message should be added for the cases we're updating its name to be one for which an action item category already exist.
3. Remove unnecessary & unused queries that were made prematurely. actionItemCategory, actionItem.

Solution

1. Add the required checks and error messages.
2. Check for unused code and remove it.

Additional details

This is a cleanup issue, which is meant to take care of some bugs and oversights that I've noticed while working on the frontend for action items.

Action Items api-issue #1595
Action Items admin-issue PalisadoesFoundation/talawa-admin#1462

[meetulr]

I would like to fix it.