tb_lock was needed when the function did retranslation. However,
since fca8a500 ("tcg: Save insn data and use it in
cpu_restore_state_from_tb") we don't do retranslation.

Get rid of the comment.
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>