Updated plugin with better commit message

Bulk PR selection removed
Code removed
content.js:
Removed enhancePullRequestList() function
Removed setupBulkSelection() function
Removed call from init() function
background.js:
Removed generateFromPRs message handler
Removed handlePRGeneration() function
style.css:
Removed .pr-toolbar styles
Removed .pr-selector styles
Removed bulk selection related CSS
Documentation:
Updated README.md to remove bulk selection references
Updated AMO_SUBMISSION_INFO.md to remove bulk selection mentions