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

 reporter_name="Julien DUSSER"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

 reporter_comment="Soft-float for the 405 PowerPC"