Remove GPG signing since we're no longer deploying to Maven central?

[landonreed]

I (and @br648) have noticed errors when running mvn package locally with gpg signing. Since we're deploying gtfs-lib to jitpack, I'm wondering if this plugin is still needed?

gtfs-lib/pom.xml

Lines 126 to 140 in a9eb787

	<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-gpg-plugin</artifactId>
	<version>1.6</version>
	<executions>
	<execution>
	<id>sign-artifacts</id>
	<!-- We sign in the verify phase, which means it will happen for install and deploy but not package. -->
	<phase>verify</phase>
	<goals>
	<goal>sign</goal>
	</goals>
	</execution>
	</executions>
	</plugin>