[SCSI] fix potential panic with proc on module removal

There's a problem in our host release in that it calls
scsi_proc_hostdir_rm(). However, if you hold a reference to the host as
you remove the module, the host template (which proc uses) will be freed
and the system will panic when the host device is finally released.

Fix this by moving scsi_proc_hostdir_rm() to where it should be: in
scsi_remove_host().

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>