Revert "[fix] sphinx 3.1 CSS issues"

This reverts commit 0616684b.

Since PR https://github.com/sphinx-doc/sphinx/pull/7878 has been merged into
Spinx-doc (v3.1.2), this patch is no longer needed:

  See sphinx-doc project, PR 7838 & 7484 with elementary patch to the basic CSS:

  - https://github.com/sphinx-doc/sphinx/issues/7838#issuecomment-646009605
  - https://github.com/sphinx-doc/sphinx/pull/7484#issuecomment-646058972



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>