Remove obsolete maven plugin version declarations

In some of the pom.xml files plugins are configured, with a version number. But that version number is already declared in the plugin management section of the project pom. The declaration is obsolete and can be removed.

From the root pom these lines can be removed:
<version>${maven.plugin.buildnumber.version}</version>
<version>${maven.plugin.jar.version}</version>

From the bootstrap/webfiles.pom this line can be removed:
<version>${maven.plugin.resources.version}</version>

This will prevent some warnings in Eclipse.