[gdb/tdep] Declare score-*-* target obsolete

I tried out the script gdb/gdb_mbuild.sh, and ran into:
...
score-elf ...
... configure --target=score-elf
... make score-elf
... run score-elf
score-elf: gdb dumped core
Terminated
...

Gdb runs into this internal error in initialize_current_architecture:
...
  if (! gdbarch_update_p (info))
    internal_error (__FILE__, __LINE__,
                    _("initialize_current_architecture: Selection of "
                      "initial architecture failed"));
...

The call to gdbarch_update_p fails because commit 575b4c29 ("gdb: Remove
support for S+core") removed support for the architecture.

Fix this by adding score-*-* to the list of obsolete targets in
gdb/configure.tgt, such that we're no longer able to build the configuration:
...
*** Configuration score-unknown-elf is obsolete.
*** Support has been REMOVED.
make: *** [Makefile:12806: configure-gdb] Error 1
...

Also remove the related line from the "Target Instruction Set Architectures"
list in gdb/MAINTAINERS, such that gdb/gdb_mbuild.sh no longer tries to build
it.