linux-user: xtensa: Remove unused constant TARGET_NR_syscall_count

linux-user: xtensa: Remove unused constant TARGET_NR_syscall_count

Currently, there is no usage of TARGET_NR_syscall_count for target
xtensa, and there is no obvious indication if there is some planned
usage in future.

CC: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1581596954-2305-10-git-send-email-aleksandar.markovic@rt-rk.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>