uclibc: remove version selection

Since we now only support uClibc-ng, remove the version selection from
the uclibc package.

Note that the BR2_UCLIBC_VERSION_SUPPORTS_* hidden booleans, which
were only used to allow each uClibc version to specify which thread
implementation they support and on which architecture are removed. Now
such architecture dependencies are directly encoded in the "Thread
library implementation" choice.

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