RFR (JDK10/JAXP) 8181151: Fix lint warnings in JAXP repo: cast

[Joe Wang]

Hi,

This change fixes about 300 [cast] warnings in the JAXP sources. Changes 
are basically removing the redundant cast, a bit more than that only in 
one case: in XSDUniqueOrKeyTraverser at line 92, uniqueOrKey was 
assigned to itself when it meant to be "idc". The change didn't affect 
the operation since idc is a result of the get operation with 
uniqueOrKey's name.

JAXP tests passed.

JBS: https://bugs.openjdk.java.net/browse/JDK-8181151
webrevs: http://cr.openjdk.java.net/~joehw/jdk10/8181151/webrev/

Thanks,
Joe