no need to strdup() and free()

for deleting backends that have been added to directors

immediately after deletion

backend.list, for example. Also remove the extra checking in delete(),
since we only delete backends created by the VMOD.

assertion

VCL

when called for a backend that has already been deleted

names of dynamic backends may begin with a digit (unlike static
backend declarations)

verifies logging the backend name (BackendOpen, -Reuse and -Close)