[5.0] Remove unused SCSS

[wilsonge]

Summary of Changes

Removes some as far as I can tell unused SCSS from the login scss. The motivation is that I'm trying to reduce the amount of places we're using var(--template-special-color);

Testing Instructions

Check the login page (including ideally with some passkey & mfa methods enabled. Ideally there should be no differences.

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

[ceford]

I have tested this item ✅ successfully on 282800f

I tested with Passkey and Code by email - no obvious differences and it worked fine.

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[brianteeman]

lgtm

[brianteeman]

I have tested this item ✅ successfully on 282800f

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[Quy]

RTC

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