[VEG-2362]: Remove unused uglifyjs-webpack-plugin

[EmilyBie]

✌️

/cc @zendesk/vegemite

Description

Describe the original problem and the changes made on this PR.

According to this PR, uglifyjs-webpack-plugin has been replaced by terser-webpack-plugin and has been no longer used any more.
Now uglifyjs-webpack-plugin is causing following errors when run npm install because of it's UglifyJS does not work with webpack v5.

npm ERR! code ERESOLVE
npm ERR! ERESOLVE could not resolve
npm ERR!
npm ERR! While resolving: [email protected]
npm ERR! Found: [email protected]
npm ERR! node_modules/webpack
npm ERR!   peer webpack@"4.x.x || 5.x.x" from @webpack-cli/[email protected]
npm ERR!   node_modules/@webpack-cli/configtest
npm ERR!     @webpack-cli/configtest@"^1.0.4" from [email protected]
npm ERR!     node_modules/webpack-cli
npm ERR!       peer webpack-cli@"4.x.x" from @webpack-cli/[email protected]
npm ERR!       3 more (@webpack-cli/info, @webpack-cli/serve, the root project)
npm ERR!   peer webpack@">=2" from [email protected]
npm ERR!   node_modules/babel-loader
npm ERR!     dev babel-loader@"^8.2.3" from the root project
npm ERR!   7 more (karma-webpack, terser-webpack-plugin, ts-loader, ...)
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer webpack@"^4.0.0" from [email protected]
npm ERR! node_modules/uglifyjs-webpack-plugin
npm ERR!   dev uglifyjs-webpack-plugin@"^2.2.0" from the root project
npm ERR!
npm ERR! Conflicting peer dependency: [email protected]
npm ERR! node_modules/webpack
npm ERR!   peer webpack@"^4.0.0" from [email protected]
npm ERR!   node_modules/uglifyjs-webpack-plugin
npm ERR!     dev uglifyjs-webpack-plugin@"^2.2.0" from the root project
npm ERR!

Tasks

• Include comments/inline docs where appropriate
• Add unit tests
• Update changelog here
• Add label from one of 'patch'/'minor'/'major' if you want to create a release when merged

References

• JIRA:

DevQA Steps

• Make sure Zendesk Apps Framework regression tests are passing with this change.
• Add specific DevQA instructions here ....

NOTE: DevQA steps are to be actioned only once code has been reviewed and approved.

Risks

• [low] Remove unused dependency

Rollback Plan

1. Quickly roll back to the prior release so that customers can refresh to resolve their issue.
2. Revert this PR to restore the master branch to a deployable green state.
3. Notify the author.