Fixes terminate_connection live migration issue

Reverts the changes to cinder/volume/manager.py added in
commit b868ae707f9ecbe254101e21d9d7ffa0b05b17d1 as calling
remove_export in terminate_connection causes Nova live
migration to fail when volumes are attached.

Change-Id: I38f3db336db22bb096a8855bcbbfeba16e517b80
Closes-Bug: #1361738