Android: remove "nativeSetComposingText" since it's not used anymore

(and it may fail registering at init if code is cleaned with proguard)

android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -2000,10 +2000,7 @@ class SDLInputConnection extends BaseInputConnection {
 
     public static native void nativeCommitText(String text, int newCursorPosition);
 
-    public native void nativeGenerateScancodeForUnichar(char c);
-
-    public native void nativeSetComposingText(String text, int newCursorPosition);
-
+    public static native void nativeGenerateScancodeForUnichar(char c);
 }
 
 class SDLClipboardHandler implements