lib: parser: remove incorrect assert

lib: parser: remove incorrect assert

A {foo|bar|baz} graph node will have more than 1/2 incoming links even
if no other references are left to it (which is what the assert was
previously trying to ensure.)

I don't see a good way to "fix" the assert so I'm just removing it.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>