Avoid the file picker adding .zip for XML exports

We were setting the MIME type of the intent to select the target export
file to application/zip. This made the resulting file end up with
.gnca.zip extension.

Author: rivaldi8

app/src/main/java/org/gnucash/android/ui/export/ExportFormFragment.java

@@ -502,7 +502,7 @@ private void selectExportFile() {
 		createIntent.setType("text/*").addCategory(Intent.CATEGORY_OPENABLE);
 		String bookName = BooksDbAdapter.getInstance().getActiveBookDisplayName();
 
-		if (mExportFormat == ExportFormat.XML || mExportFormat == ExportFormat.QIF) {
+		if (mExportFormat == ExportFormat.QIF) {
 			createIntent.setType("application/zip");
 		}