[5.0] Remove unused SCSS from cpanel file

[wilsonge]

Summary of Changes

This removes some as far as I can tell unused SCSS from the cpanel module. I cannot find any references to .cpanel-add-module-icon-help, .com-cpanel-help, .cpanel-add-module-icon-system or .com-cpanel-system inside core.

Additionally when viewing the help page in the main menu or any of the cpanel dashboards I can't see any visually differences changes after applying this

Testing Instructions

Validate classes don't exist in core and all cpanel views (primary dashboard, system dashboard and help page) are unchanged after applying this PR

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[brianteeman]

I have tested this item ✅ successfully on 0f28b1b

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[Quy]

I have tested this item ✅ successfully on 0f28b1b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41993.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41993.}