Remove `docker stack ps -a` to match removal of `docker service/node ps -a`

In #28507 and #28885, `docker service/node ps -a` has been removed so that
information about slots are show up even without `-a` flag.

The output of `docker stack ps` reused the same output as `docker service/node ps`.
However, the `-a` was still there. It might make sense to remove `docker stack ps -a`
as well to bring consistency with `docker service/node ps`.

This fix is related to #28507, #28885, and #25983.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/commandline/stack_ps.md

@@ -21,10 +21,10 @@ Usage:  docker stack ps [OPTIONS] STACK
 List the tasks in the stack
 
 Options:
-  -a, --all            Display all tasks
-  -f, --filter value   Filter output based on conditions provided
-      --no-resolve     Do not map IDs to Names
-      --no-trunc       Do not truncate output
+  -f, --filter filter   Filter output based on conditions provided
+      --help            Print usage
+      --no-resolve      Do not map IDs to Names
+      --no-trunc        Do not truncate output
 ```
 
 Lists the tasks that are running as part of the specified stack. This