agent: Fix concurrent remove.
agent: Fix concurrent remove.

There's no point to remove the task from `a.remove` since the removal
goroutine is already taking care of that.

Fixes #710

Signed-off-by: Andrea Luzzardi <442ffe3ffbfd024afcf0b7f8b108d05bca3aae43@gmail.com>

jlhawn/swarmkit,docker/swarmkit,allencloud/swarmkit,cheyang/swarmkit,runshenzhu/swarmkit,alyyousuf7/swarmkit,diogomonica/swarmkit,jlhawn/swarmkit,runshenzhu/swarmkit,yp-engineering/swarmkit,cezarsa/swarmkit,RenaudWasTaken/swarmkit,deljuven/swarmkit,AliyunContainerService/swarmkit,docker/swarmkit,vasil-yordanov/swarmkit,deljuven/swarmkit,fate-grand-order/swarmkit,cezarsa/swarmkit,elifa/swarmkit,cheyang/swarmkit,elifa/swarmkit,yongtang/swarmkit,yongtang/swarmkit,dperny/swarmkit-1,aaronlehmann/swarmkit,RenaudWasTaken/swarmkit,yp-engineering/swarmkit,diogomonica/swarmkit,sungwonh/swarmkit,Noyan759/swarmkit,sungwonh/swarmkit,allencloud/swarmkit,aaronlehmann/swarmkit,dperny/swarmkit-1,alyyousuf7/swarmkit,vasil-yordanov/swarmkit,fate-grand-order/swarmkit,AliyunContainerService/swarmkit,Noyan759/swarmkit