[DOC] Remove tox.ini doppelganger

[DOC] Remove tox.ini doppelganger

in docs folder

- same profiles are declared in tox.ini.
  They can be called from the docs folder without it.
- once run from the docs folder, this generates invalid files
  in doc/.tox for linters setup (e.g.  docs/.tox/[..]/invalid.rst).
  They can cause warnings treated as errors by root tox profiles.

Issue-ID: DOC-692
Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I473ae1193c1e5b124c8532a1b8185f01c67c70de
(cherry picked from commit 406a9c94549c014fab8d777a788832f3a9197703)