default-configs/aarch64-linux-user.mak: Remove unused define

The uses of the CONFIG_GDBSTUB_XML define were removed in commit
b77abd95, but the define in aarch64-linux-user.mak somehow
escaped the cull (the patchset probably crossed in the mail with
the patches adding aarch64 support). Remove the stray define.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 1447690178-4560-1-git-send-email-peter.maydell@linaro.org