tox: Drop redundant injection of VIRTUAL_ENV variable

The environment variable is set automatically.

cf. https://github.com/tox-dev/tox/commit/be5373bab

Change-Id: Ibe1c52031ddf6a241de755b430ac71197b9a049a
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
1 file changed