libc/glibc: Remove redundant LIBC_GLIBC_TARBALL config option

Unused since January 2010.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
Message-Id: <7a666ba8c0ea5e8f4b18.1349931190@localhost.localdomain>
PatchWork-Id: 190783

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }