Partially revert "XenAPI: Monitor the GC when coalescing"

This partially reverts commit 270d4f1d6b100c802a65b31d35e406528aa7fd27.

The plugin has been left so the plugin version is not made incompatible
by removing a call, but the call to it from vm_utils has been removed.

I expect this call will be re-introduced in future when we have identified
the cause of the race condition and the appropriate usage.

The original fix was made to fix bug 1258169 where the number of attempts
made were not sufficient, so this revert does not reduce the number of
attempts.

Change-Id: I473b81b9970990b877d1886bb28a96888cc05f98
Closes-bug: 1282822
Merge "Partially revert "XenAPI: Monitor the GC when coalescing""