Format pom.xml files

This format commit fixes whitespace/tab indentation inconsistencies.
Consistent indentation is now **two whitespaces**.

This commit also adds xml schemes to the <project> tags.
Remove version of maven-clean-plugin

Is now managed by parent pom since we moved to net.imagej.pom-imagej.
Do not import packages

Import-Package was never used in previous build system manifests. Instead
we use Require-Bundle to "import" all packages of a bundle.