Revert some modification of PR #9435 to make show the usage of docker exec

Signed-off-by: Lei Jitang <leijitang@huawei.com>
PR #9435 modify the function `ParseExec` in `runconfig/exec.go` to check if there is 
enough arguments to create exec command,but this cause a problem that the usage of 
of docker exec would never show if the user enter the wrong format of command or just want to 
see the usage with `docker exec`,so this is weird. Actually,I don't think there is need to check if 
there is enough arguments in `ParseExec`,because the client API  'CmdExec' will do  that and show the usage if the arguments is wrong.
This patch is just a revert of the modification of function `ParseExec` made by PR#9435