Delete work-around for Ant 1.8 in Gradle build

Ant 1.9.2 is packaged with Gradle since release 1.10. Since the Spring
Framework build now uses Gradle 1.11, there is no longer a need for the
"javac1.7" build compiler work-around for the spring-oxm module.

build.gradle

@@ -438,11 +438,6 @@ project("spring-oxm") {
 	description = "Spring Object/XML Marshalling"
 	apply from: "oxm.gradle"
 
-	// The following is a work-around until the Gradle build uses
-	// Ant 1.9.x by default. This is necessary to avoid the genCastor
-	// "Class not found: javac1.8" issue with Ant versions prior to 1.9.x
-	ant.properties["build.compiler"] = "javac1.7"
-
 	compileTestJava {
 		// necessary to avoid java.lang.VerifyError on jibx compilation
 		// see http://jira.codehaus.org/browse/JIBX-465