getting-started: remove the last 2 references to JCenter

docs/modules/ROOT/pages/getting-started/build_from_sources.adoc

@@ -76,7 +76,7 @@ ext.jmeVersion = "[3.2,)"
 
 repositories {
     mavenLocal()
-    jcenter()
+    mavenCentral()
 }
 
 dependencies {

docs/modules/ROOT/pages/getting-started/simpleapplication_from_the_commandline.adoc

@@ -58,14 +58,11 @@ After cloning, open the `build.gradle` file and uncomment the `mavenLocal()` rep
 ----
 repositories {
     //This is where jme3 dependencies are stored.
-    jcenter()
+    mavenCentral()
 
     //Uncomment this if you install local dependencies.
     mavenLocal()
 
-    //Uncomment this if you use external dependencies
-    //mavenCentral()
-
     //Uncomment this if you use jme3-niftygui
     //maven{url 'http://nifty-gui.sourceforge.net/nifty-maven-repo'}