softmmu: remove useless condition in watchpoint check

cpu_check_watchpoint function checks cpu->watchpoint_hit at the entry.
But then it also does the same in the middle of the function,
while this field can't change.
That is why this patch removes this useless condition.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <163542169094.2127597.8801843697434113110.stgit@pasha-ThinkPad-X280>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>