This rule is defined by the following Java class: net.sourceforge.pmd.rules.strings.UnnecessaryCaseChange
Example:
public class Foo { public boolean bar(String buz) { // should be buz.equalsIgnoreCase("baz") return buz.toUpperCase().equals("baz"); // another unnecessary toUpperCase() // return buz.toUpperCase().equalsIgnoreCase("baz"); } }