cpus-common: remove redundant call to exclusive_idle()

No need to call exclusive_idle() from cpu_exec_end since it is done
immediately afterwards in cpu_exec_start.  Any exclusive section could
run as soon as cpu_exec_end leaves, because cpu->running is false and the
mutex is not taken, so the call does not add any protection either.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
