[gdb/build] Mark ms1 as deleted in MAINTAINERS

[gdb/build] Mark ms1 as deleted in MAINTAINERS

When running gdb/gdb_mbuild.sh, we run into:
...
ms1-elf ...
... src/configure --target=ms1-elf --enable-gdb-build-warnings=,-Werror \
  --enable-sim-build-warnings=,-Werror
... make -k -j 5 ms1-elf
ms1-elf: compile failed
...

In mbuild/ms1-elf/Build.log we find:
...
*** Configuration mt-unknown-elf is obsolete.
*** Support has been REMOVED.
make: *** [Makefile:10995: configure-gdb] Error 1
...

The support for ms1-elf, aka mt-elf, has been removed since commit dc71152484
"Remove mt port" in Jan 2018.  Make this official in the MAINTAINERS file.

Tested using gdb/gdb_mbuild.sh.

gdb/ChangeLog:

2020-08-11  Tom de Vries  <tdevries@suse.de>

	* MAINTAINERS: Mark ms1 as deleted.