POM: remove superfluous license-maven-plugin block

This plugin has a groupId of org.codehaus.mojo, which is one of the two
groupIds searched by default when doing Maven plugin prefix resolution.
So there's no need to declare it just for that purpose.

See:
http://maven.apache.org/guides/introduction/introduction-to-plugin-prefix-mapping.html#Configuring_Maven_to_Search_for_Plugins
Add scijava-maven-plugin to pluginManagement

It was a definite oversight that it was not already there...
Make nar: and scijava: plugin prefixes work

Otherwise, because they do not use one of the two blessed groupIds,
Maven fails to resolve them from their prefix unless the project POM
declares them itself in its <plugins> section.

See:
http://maven.apache.org/guides/introduction/introduction-to-plugin-prefix-mapping.html#Configuring_Maven_to_Search_for_Plugins