remove curious hard-coded yangtools 2.1.8 which seems no longer needed

remove curious hard-coded yangtools 2.1.8 which seems no longer needed

This appears to have been introduced in
I417b22698df04edc8263b57e6fcd5d0a558c6ef4, but with the subsequent "Bump
to mdsal 3.0.6" in I6bbb6fa8af6860d9da08fcdde165a43c4abc6998 (and
"Finish bumping to yangtools 2.1.8" in
I51f7a2b30d40ae2b694df000465d383d165d9a0d) can be removed, given that
mdsal 3.0.6 already pulls yangtools 2.1.8.

Change-Id: Iaefc1c066b13aa7181c93e1f2963d798c43f121b
Signed-off-by: Michael Vorburger <vorburger@redhat.com>