Remove jquery.quicksearch.js file

[htdat]

Fix #661

Description

The reason for this PR is mentioned in #661.

What I've done to do this cleanup:

• Searched jquery.quicksearch.js and removed the relevant places.
• Searched jquery-quicksearch and removed the relevant things.
• Searched quicksearch and removed the relevant things. After this PR, there are still few places for this keyword quicksearch in modules/notifications/lib/notifications.css and modules/user-groups/lib/user-groups.css but I think we should keep it.

Steps to Test

• Apply this PR.
• Apply the diff in the Note section in this PR https://github.com/Automattic/Edit-Flow/pull/649
• Open browser console and watch for errors.
• Check across all Edit Flow places as mentioned in PR 649.
• Focus on two places that are mostly affected by this PR: (1) Notifications section when editing a post, and (2) WP Admin > Edit Flow > User Groups > Edit a group, where user groups display.

[htdat]

Looks like all PHP tests failed for the same reason as mentioned here https://github.com/Automattic/Edit-Flow/pull/667#issuecomment-859139939

I think eventually we may run phpcbf . --standard=WordPress, which is similar to what @nielslange has done for CAP https://github.com/Automattic/Co-Authors-Plus/pull/771

[ingeniumed]

Opened #733 to do this, so closing this off