gradle: We only need to exclude xpp3_min

Since xpp3 is only declared as 'implementation' in
smack-xmlparser-xpp3, whereas xpp3_min is declared as 'api'. This
means that only xpp3_min is pulled in by the build system, but as it
is already provided by Android, we need to exclude it again.

build.gradle

@@ -37,7 +37,6 @@ allprojects {
         // Exclude XmlPullParser from Smack dependencies, as its now provided by Android
         // https://stackoverflow.com/questions/48488563/gradle-xpp3-error/48746294#48746294
         all {
-            exclude group: 'xpp3', module: 'xpp3'
             exclude group: 'xpp3', module: 'xpp3_min'
         }
     }