Re-parent from pom-fiji to pom-trakem2

The Fiji projects saw a recent push to clean up the project
configurations, in particular to provide Reproducible Builds. In this
light, the TrakEM2-related projects should reuse pom-trakem2 as Bill of
Materials.

For further rationale, see http://fiji.sc/Reproducible_builds and
http://fiji.sc/Bill_of_Materials, respectively.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>