Remove grunt-usemin

[shamrickus]

What does this PR (Pull Request) do?

• This PR is not related to any Issue

This removes the unused and unmaintained library grunt-usemin

Which Traffic Control components are affected by this PR?

• Traffic Portal

What is the best way to verify this PR?

Run the integration tests.
Ensure production and dev builds work, as well as CiaB.

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

Diff output from running grunt dist:

❯ diff -dr dist dist_master
diff --color -dr dist/public/index.html dist_master/public/index.html
34,37c34,37
<         <link rel="stylesheet" media="all" href="resources/styles/theme.css?built=1595342124350">
<         <link rel="stylesheet" media="all" href="resources/styles/loading.css?built=1595342124350">
<         <link rel="stylesheet" media="all" href="resources/styles/main.css?built=1595342124350">
<         <link rel="stylesheet" media="all" href="resources/assets/css/custom.css?built=1595342124350">
---
>         <link rel="stylesheet" media="all" href="resources/styles/theme.css?built=1595342071418">
>         <link rel="stylesheet" media="all" href="resources/styles/loading.css?built=1595342071418">
>         <link rel="stylesheet" media="all" href="resources/styles/main.css?built=1595342071418">
>         <link rel="stylesheet" media="all" href="resources/assets/css/custom.css?built=1595342071418">
51c51
<         <script src="resources/assets/js/shared-libs.js?built=1595342124350"></script>
---
>         <script src="resources/assets/js/shared-libs.js?built=1595342071418"></script>
53,54c53,54
<         <script src="resources/assets/js/app.js?built=1595342124350"></script>
<         <script src="resources/assets/js/config.js?built=1595342124350"></script>
---
>         <script src="resources/assets/js/app.js?built=1595342071418"></script>
>         <script src="resources/assets/js/config.js?built=1595342071418"></script>

[mitchell852]

@shamrickus - do you need to update package-lock.json as well?

[shamrickus]

Yes, good point. I will update that

[mitchell852]

retest this please

[mitchell852]

rebuilt TP dependencies and TP and ran TP tests. all looks good.