Fix Pod Log topbar position when perf bar is disabled

What does this MR do?

• Fix sticky topbar position when perf bar is disabled.
• Move perf-bar CSS to mixin, remove unneeded var
• remove unused .affix styles, we are using position: sticky instead of bs affix

CE Backport

What are the relevant issue numbers?

Closes #7203 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder