RFR (JDK 13/java.xml) 8186321: Cleanup: SourceTreeManager not used

[Joe Wang]

Please review a cleanup of an unused class. All core and xml tests passed.

JBS: https://bugs.openjdk.java.net/browse/JDK-8186321
webrevs: http://cr.openjdk.java.net/~joehw/jdk13/8186321/webrev/

Thanks,
Joe