core: remove show-dependency-tree

show-dependency-tree was introduced in this release cycle, as a way to
quickly and easily provide the dependency tree to graph-depends.

show-dependency-tree is no longer used, now that graph-depends has been
switched over to using the more versatile show-info.

Beside, show-dependency-tree has never been part of a release.

Drop it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>