Remove redundant project.build.sourceEncoding property (#7011)

pom.xml

@@ -77,10 +77,6 @@ THE SOFTWARE.
     <revision>2.365</revision>
     <changelist>-SNAPSHOT</changelist>
 
-    <!-- *.html files are in UTF-8, and *.properties are in iso-8859-1, so this configuration is actually incorrect,
-    but this suppresses a warning from Maven, and as long as we don't do filtering we should be OK. -->
-    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-
     <!-- configuration for patch tracker plugin  -->
     <project.patchManagement.system>github</project.patchManagement.system>
     <patch.request.organisation>jenkinsci</patch.request.organisation>