Do not show warning message with xlf output format

[davidlemaitre]

Q	A
Bundle version	1.3.2
Symfony version	2.8.33
PHP version	7.0.26

Expected behavior

Do not show warning message for translation files with xlf file extension

Actual behavior

The warning message disappears if xliff file extension is used for translation files but keeps showing for xlf file extension.

Steps to reproduce

Use xlf output format and go to Web UI