Remove dead code in ide.scss

What does this MR do?

Fixes some of the cases in: #27774 (closed)

1. isZoomable and isZoomed were removed / altered in https://gitlab.com/gitlab-org/gitlab/blob/8a23bcc9bc0f7ab453ee09d41a9407d40d57ba4c/app/assets/javascripts/vue_shared/components/content_viewer/viewers/image_viewer.vue.

2. .file-content class was removed from image_viewer in https://gitlab.com/gitlab-org/gitlab/blob/f7df9ddb52be8a03b8fbd8c9a870f3e3af577562/app/assets/javascripts/vue_shared/components/content_viewer/viewers/image_viewer.vue, so now the file-content class is only present in download_viewer, and it doesn't contain any img.

3. .multi-file-discard-btn was removed in 24c84ea6.

Screenshots

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

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• 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