uclibc: remove Xtensa specific version

Since we now have uClibc-ng, and it is used as the default uClibc
version, there is no real need to keep the Xtensa version: this
version is just a specific Git revision of uClibc master branch, and
uClibc-ng is also based on uClibc master branch.

In addition, since in the choice of uClibc versions, uClibc-ng is
already *before* the Xtensa specific version, uClibc-ng has in fact
already been the default uClibc version for Xtensa since uClibc-ng
support was introduced.

See for example:

  http://autobuild.buildroot.org/results/c85/c854fae31d82e0f11d5a8b5bfc196c29bd7e21db/config



for a build on Xtensa with the internal toolchain, where in fact
uClibc-ng is used instead of the Xtensa specific version.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>