Suggestion to shorten pom, and skip needless maven-gpg-plugin execution

When I builded this project, I found that maven-gpg-plugin requests me to input GPG passphrase.
I felt little troublesome, because what I need is not deploy but just building product.

The [oss-parent project](https://github.com/sonatype/oss-parents/blob/oss-parent-7/pom.xml) provides `sonatype-oss-release` profile to sign jar file, and this profile will be activated when we execute `maven-release-plugin` like `mvn release:prepare`. So we do not have to make original profile.

See also: http://lacostej.blogspot.jp/2011/01/publishing-your-signed-oss-project-to.html