mysql: remove stray comment

In commit 9863553f ("packages: all
salute the passing of avr32"), the dependency on !BR2_avr32 was
removed, but not the comment above it. So now, the comment seems to
apply to a line selecting some library, which doesn't make sense. This
commit gets rid of the unneeded and confusing comment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>