Moved docker-enter function to docker plugin

Using the recommended enter approach (exec) instead of the previously
used nsenter.

plugins/available/boot2docker.bash

@@ -7,9 +7,4 @@ if [[ `uname -s` == "Darwin" ]]; then
   export DOCKER_HOST="tcp://192.168.59.103:2376"
   export DOCKER_CERT_PATH="~/.boot2docker/certs/boot2docker-vm"
   export DOCKER_TLS_VERIFY=1
-
-  docker-enter() {
-    boot2docker ssh '[ -f /var/lib/boot2docker/nsenter ] || docker run --rm -v /var/lib/boot2docker/:/target jpetazzo/nsenter'
-    boot2docker ssh -t sudo "/var/lib/boot2docker/docker-enter \"$1\""
-  }
 fi