classes: image: Set empty weak default IMAGE_LINGUAS

This is already set in default-distrovars.inc and so this assignment
will almost never succeed. Rather than leaving it to confuse, set an
empty weak default.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>