Docs: remove reference to detox

Docs: remove reference to detox

Detox globaljjb support was removed in a previous commit.
1873622e1b5bf98150db6850901d190dd6c35422
Parallel jobs are now natively supported by tox since version 3.7.0
thanks to the option "-p" / "--parallel".
This new option offers more possibilities than detox and other options
have also been introduced to tune tox behavior in parallel mode.
The configuration choices made in globaljjb must be clarified.

Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I82c88ea96ee1332b95ab13f0b7dbf2a04e00f37a