PMD: Discontinue using Rule name

category/java/errorprone.xml/UselessOperationOnImmutable as it is
scheduled for removal from PMD.

PMD 8.0.0 will remove support for this Rule.

Author: garydgregory

src/site/resources/pmd/pmd-ruleset.xml

@@ -59,7 +59,6 @@
   <rule ref="category/java/errorprone.xml/UnconditionalIfStatement"/>
   <rule ref="category/java/errorprone.xml/UnnecessaryConversionTemporary"/>
   <rule ref="category/java/errorprone.xml/UnusedNullCheckInEquals"/>
-  <rule ref="category/java/errorprone.xml/UselessOperationOnImmutable"/>
   <rule ref="category/java/multithreading.xml/AvoidThreadGroup"/>
   <rule ref="category/java/multithreading.xml/DontCallThreadRun"/>
   <rule ref="category/java/multithreading.xml/DoubleCheckedLocking"/>