(BC) Remove js index file

[luigifab]

Description

This PR require #1618 because it remove the last usage of:

<script type="text/javascript" src="<?php echo $this->getJsUrl() ?>index.php/x.js?f=prototype/prototype.js,...">

I think it's so outdated.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)

[jouriy]

Would love to see it in 1.9.4.x as well, removing it will allow to have whole js/ static with legit "php_flag engine 0" in js/.htaccess and default nginx config serving js/ as static files.

[luigifab]

@jouriy this PR is a BC for people that use this old functionality, so I changed to 20.0.

[jouriy]

@luigifab, great, I can keep it on my own, so no problem

[github-actions]

Unit Test Results

1 files  ±0  1 suites  ±0   0s ⏱️ ±0s
0 tests ±0  0 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
7 runs  +1  7 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit f93c9e0. ± Comparison against base commit ff6189a.