Remove db-access semaphore in ML2
Remove db-access semaphore in ML2

With the addition of the deadlock retry decorator in
I635cc49ca69f589f99ab145d4d51e511b24194d2 and the change
to instantaneous deadlock feedback under the pymysql driver,
this semaphore should no longer be necessary. Removing it
will allow us to benefit from processing multiple requests
simultaneously.

Change-Id: If5dc76a2974d13d45f0cc88419bcccb3332576cf

eonpatapon/neutron,mahak/neutron,SamYaple/neutron,paninetworks/neutron,SamYaple/neutron,mattt416/neutron,glove747/liberty-neutron,chitr/neutron,mmnelemane/neutron,sebrandon1/neutron,shahbazn/neutron,asgard-lab/neutron,takeshineshiro/neutron,MaximNevrov/neutron,cloudbase/neutron,eayunstack/neutron,dhanunjaya/neutron,javaos74/neutron,apporc/neutron,neoareslinux/neutron,neoareslinux/neutron,yanheven/neutron,swdream/neutron,wolverineav/neutron,openstack/neutron,shahbazn/neutron,huntxu/neutron,JianyuWang/neutron,huntxu/neutron,JianyuWang/neutron,bigswitch/neutron,dims/neutron,MaximNevrov/neutron,suneeth51/neutron,watonyweng/neutron,swdream/neutron,mahak/neutron,vivekanand1101/neutron,sasukeh/neutron,skyddv/neutron,jumpojoy/neutron,eayunstack/neutron,mattt416/neutron,adelina-t/neutron,klmitch/neutron,wenhuizhang/neutron,cisco-openstack/neutron,wenhuizhang/neutron,mmnelemane/neutron,cisco-openstack/neutron,jumpojoy/neutron,barnsnake351/neutron,glove747/liberty-neutron,jacknjzhou/neutron,openstack/neutron,skyddv/neutron,sasukeh/neutron,sebrandon1/neutron,vivekanand1101/neutron,suneeth51/neutron,mahak/neutron,klmitch/neutron,igor-toga/local-snat,cloudbase/neutron,yanheven/neutron,watonyweng/neutron,dhanunjaya/neutron,dims/neutron,adelina-t/neutron,apporc/neutron,noironetworks/neutron,asgard-lab/neutron,eonpatapon/neutron,silenci/neutron,jacknjzhou/neutron,silenci/neutron,noironetworks/neutron,igor-toga/local-snat,openstack/neutron,javaos74/neutron,paninetworks/neutron,chitr/neutron,takeshineshiro/neutron,bigswitch/neutron,barnsnake351/neutron,wolverineav/neutron