accel/tcg: Remove dead code from rr_cpu_thread_fn()

accel/tcg: Remove dead code from rr_cpu_thread_fn()

The main loop in rr_cpu_thread_fn() can never terminate, so the
code at the end of the function to clean up the RCU subsystem is
dead code. Replace it with g_assert_not_reached().

(This is different from the other cpu_thread_fn for e.g. MTTCG or
for the KVM accelerator -- those can exit, if the vCPU they
are responsible for is unplugged. But the RR cpu thread fn
handles all CPUs in the system in a round-robin way, so even
if one is unplugged it keeps looping.)

Resolves: Coverity CID 1547782
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240815143634.3413679-1-peter.maydell@linaro.org