RFR: JDK-8055189: Cleanup gensrc after source code restructure

[Erik Joelsson]

Please review this little cleanup patch for Gensrc*.gmk files. 
GensrcProviders.gmk and GensrcJDWP are both short and are only used by 
Gensrc-jdk.jdi.gmk, so I inlined them in that file. 
Gensrc-jdk.attach.gmk was no longer doing anything so it could be removed.

I could not find any other simple inline opportunity.
Bug: https://bugs.openjdk.java.net/browse/JDK-8055189
Webrev: http://cr.openjdk.java.net/~erikj/8055189/webrev.jdk.01/

/Erik