Remove extra plugins repository

This repository is discouraged on Maven Central, and furthermore nearly
all the SciJava based projects do not need it. And if they do, they can
declare it themselves, of course; e.g.:

https://github.com/uw-loci/wiscscan-java/commit/9f0dcd4359fadba0e5afbd52cc2cf7e48f32410b#diff-600376dffeb79835ede4a0b285078036R132
Tweak whitespace