gdb/features: remove rx.xml from XMLTOC list

gdb/features: remove rx.xml from XMLTOC list

When trying to run `make` in the features directory, in a clean repo, we
get:

    Makefile:254: warning: overriding recipe for target 'rx.c'
    Makefile:250: warning: ignoring old recipe for target 'rx.c'
    make: Nothing to be done for 'all'.

The warnings come from the fact that `rx.xml` is present in two lists,
causing two `rx.c` targets to be defined.  It is ok for it to be in the
FEATURES_XMLFILES list, as this architecture uses the "feature-based"
target-descriptions.  It shouldn't be in the XMLTOC list, as this is for
architectures that define complete/static target descriptions as XML
files.

gdb/ChangeLog:

	* features/Makefile (XMLTOC): Remove rx.xml.

Change-Id: Iada4ab54b3d4542588fac543d16ee35a92537319