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 Fix jQuery warnings as WordPress upgraded it from 1.12.4-wp to 3.5.1 #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 #667 (comment)

I think eventually we may run phpcbf . --standard=WordPress, which is similar to what @nielslange has done for CAP Automattic/co-authors-plus#771

[ingeniumed]

Opened #733 to do this, so closing this off