signal/blackfin: Remove pointless UID16_SIGINFO_COMPAT_NEEDED

Nothing tests this define so just remove it.

I suspect the intention was to make the uid field in siginfo 16bit
however I can't find any code that ever tested this defined, and
even if it did it the layout has been this way for 8 years so
changing it now would break the ABI with userspace.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>