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/conf/pmd-ruleset.xml

@@ -56,7 +56,6 @@
   <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" />