media: tuner-core: Remove unused #define PREFIX

Commit 680d87c0

 ("[media] tuner-core: use pr_foo, instead of
internal printk macros") removed the use of PREFIX, remove the #define

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>