[devmgr] Don't remove already-removed device

In cases when a device has just been removed (and is flagged
as DEV_CTX_DEAD), its devhost might send it one or more last
messages.  In these cases, the debug log error "cannot
remove device twice!" is shown, which is likely to mislead
driver developers (at least it did for me).

This change suppresses a device removal that would be
initiated by the DevMgr (when Device RPC Read returns
error), if dev->flags already contains DEV_CTX_DEAD.

Test: Build, CQ

Change-Id: I1c27c89dc7dff7715a05e1e68e076dc7be6542b3