Remove db connection string env variable from tox.ini
Remove db connection string env variable from tox.ini

It is not needed with oslo.db 1.12.0.

Change-Id: I49dc450d6bb84e5a75ef9ecdf248eac567c3442c

ionutbalutoiu/ironic,pshchelo/ironic,bacaldwell/ironic,openstack/ironic,bacaldwell/ironic,hpproliant/ironic,NaohiroTamura/ironic,dims/ironic,redhat-openstack/ironic,devananda/ironic,naterh/ironic,openstack/ironic,pshchelo/ironic,dims/ironic,SauloAislan/ironic,NaohiroTamura/ironic,SauloAislan/ironic,Tan0/ironic,ionutbalutoiu/ironic