Fix markdown table header and table content borders

What does this MR do?

1. Removes :not(js-syntax-highlight) pseudo-class from markdown table styles defined in typography.scss since this breaks markdown table styles across the application. This change rollbacks a fix for syntax highlighting in https://gitlab.com/gitlab-org/gitlab-ce/commit/f3b9eda913b49649634fb82902cf4c55090260ef#b9e6a7a37b1dfbba76fb127ff44485669747bbf4_147_146.
2. Remove inclusion of md-typography (typography.scss) mixin in notes.scss to decrease the priority and specificity of markdown styles in the notes stylesheet and fix syntax highlighting used in code suggestions. This is an alternative fix to syntax highlighting.

Before

After

What are the relevant issue numbers?

Closes #57655 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer