gdb: Remove support for S+core

GCC removed support for score back in 2014 already.  Back then, we
basically agreed about removing it from GDB too, but it ended up being
forgotten.  See:

 https://sourceware.org/pipermail/gdb/2014-October/044643.html

Following through this time around.

Change-Id: I5b25a1ff7bce7b150d6f90f4c34047fae4b1f8b4