Fix always visible sidebar TODO button spinner

What does this MR do?

Fixes a regression introduced in !41132 (merged) that makes the loading spinner of the "Add TODO" button in the sidebar to be always visible (see screenshots section). See more information in #246502 (closed) as well.

note: I spent considerable time verifying what parts of the sidebar are affected by the styles removed on this Merge Request, and I can confirm that those styles only affect the loading icon in the sidebar’s TODO button.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #246502 (closed)