RFR (XXS) 8224790: Remove Xusage.txt file

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8224790
webrev: http://cr.openjdk.java.net/~dholmes/8224790/webrev/

Before Java 7 the Xusage.txt file was used to print the "java -X" help 
information in English. From Java 7 this information was provided in 
localised format by the launcher. The Xusage.txt file was not removed at 
the time because it was also used by the gamma launcher. But the gamma 
launcher was removed in java 8 - JDK-8008772. The information in 
Xusage.txt has become stale and incomplete (despite several edits post 
Java 7). It should just be removed.

Also removed the build logic that copied the file into the JDK image.

Thanks,
David