Item not deleting from SQlite Database

In my app I have multiple markers that are added to the map and saved in SQLite, but now i want to delete a specific marker when tapping on that marker and select "delete marker" So what I have done is this:

In my LocationsDB.class

@Override
public void onCreate(SQLiteDatabase db) {
    String sql = "create table " + DATABASE_TABLE + " ( " +
                 FIELD_ROW_ID + " integer primary key autoincrement , " +
                 FIELD_LNG + " double , " +
                 FIELD_LAT + " double , " +
                 FIELD_ZOOM + " text , " +
                 FIELD_TITLE + " text , " +
                 FIELD_SNIPPET + " text , " +
                 " ) ";

    db.execSQL(sql);
}

public long insert(ContentValues contentValues){
    long rowID = mDB.insert(DATABASE_TABLE, null, contentValues);
    return rowID;
}

public int del(int id){
    int cnt = mDB.delete(DATABASE_TABLE, FIELD_ROW_ID+"="+id, null);
    return cnt;
}

Then in my LocationsContentProvider.class:

 @Override
public int update(Uri uri, ContentValues values, String selection,
    String[] selectionArgs) {
    return 0;
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
    int cnt = 0;
    cnt = mLocationsDB.del(cnt);
    return cnt;
}
@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {

    if(uriMatcher.match(uri)==LOCATIONS){
        return mLocationsDB.getAllLocations();
    }
    return null;
}

@Override
public String getType(Uri uri) {
    return null;
}
}

And then finally in my MainActivity.class:

public void onClick(DialogInterface arg0, int arg1) {
    marker.remove();
    LocationDeleteTask deleteTask = new LocationDeleteTask();
    deleteTask.execute();
        }
    })

....

@Override
public void onLoaderReset(Loader<Cursor> arg0) {
}
class LocationInsertTask extends AsyncTask<ContentValues, Void, Void>{
    @Override
    protected Void doInBackground(ContentValues... contentValues) {
        getContentResolver().insert(LocationsContentProvider.CONTENT_URI, contentValues[0]);
        return null;
    }
}

private class LocationDeleteTask extends AsyncTask<Void, Void, Void>{

@Override
protected Void doInBackground(Void... params) {
   getContentResolver().delete(LocationsContentProvider.CONTENT_URI, null, null);
   return null;

This initially works when selecting "delete marker" but when returning to the activity the marker then returns and doesn't get deleted?

Am not sure what i am doing wrong, so hoping someone could help me please?