bitbake: toaster: layerdetails Update implementation of delete imported layer

bitbake: toaster: layerdetails Update implementation of delete imported layer

Update the implementation of delete an imported layer so that it is
consistent with the other delete messages and wording. Also use the new
libtoaster way of setting a notification that the delete was successful.

(Bitbake rev: 0b8d3ac48b5a0984963d664ff5630e3b02c4ecd1)

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>