uclibc: remove support for the snapshot version

The snapshot version points to the original uClibc project, which is
dead. Moreover, we no longer support "snapshot" versions for any other
Buildroot component, so there is no reason to keep it for uClibc.

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