Don't run jenv javahome twice

The second call to "jenv javahome" to set JDK_HOME is unnecessary. It
adds 60ms on my system.

available-plugins/export/etc/jenv.d/init/export_jenv_hook.bash

@@ -7,7 +7,7 @@
 
   if [ -e "$JAVA_HOME/bin/javac" ]
   then
-    export JDK_HOME=$(jenv javahome)
+    export JDK_HOME="$JAVA_HOME"
     export JENV_FORCEJDKHOME=true
   fi
    }
@@ -17,4 +17,4 @@ if ! [[ "$PROMPT_COMMAND" =~ _jenv_export_hook ]]; then
 fi
 
 
-_jenv_export_hook
+_jenv_export_hook