docs: removed links to stack_tasks & stack_ls

`docsreference/commandline/stack_tasks.md ` &  `docsreference/commandline/stack_ls.md` do not exist.

I need this to be merge to solve that issue: https://github.com/docker/docker.github.io/issues/1068, and merge new CI tests for the docs: https://github.com/docker/docker.github.io/pull/1052

Signed-off-by: Adrien Duermael <adrien@duermael.com>

docs/reference/commandline/deploy.md

@@ -52,4 +52,3 @@ axqh55ipl40h  vossibility-stack_vossibility-collector  1 icecrime/vossibility-co
 * [stack config](stack_config.md)
 * [stack deploy](stack_deploy.md)
 * [stack rm](stack_rm.md)
-* [stack tasks](stack_tasks.md)

docs/reference/commandline/stack_config.md

@@ -26,4 +26,3 @@ Displays the configuration of a stack.
 * [stack rm](stack_rm.md)
 * [stack services](stack_services.md)
 * [stack ps](stack_ps.md)
-* [stack ls](stack_ls.md)

docs/reference/commandline/stack_deploy.md

@@ -55,4 +55,3 @@ axqh55ipl40h  vossibility-stack_vossibility-collector  1 icecrime/vossibility-co
 * [stack rm](stack_rm.md)
 * [stack services](stack_services.md)
 * [stack ps](stack_ps.md)
-* [stack ls](stack_ls.md)

docs/reference/commandline/stack_rm.md

@@ -29,4 +29,3 @@ a manager node.
 * [stack deploy](stack_deploy.md)
 * [stack services](stack_services.md)
 * [stack ps](stack_ps.md)
-* [stack ls](stack_ls.md)

docs/reference/commandline/stack_services.md

@@ -60,4 +60,3 @@ The currently supported filters are:
 * [stack deploy](stack_deploy.md)
 * [stack rm](stack_rm.md)
 * [stack ps](stack_ps.md)
-* [stack ls](stack_ls.md)