config-linux: Remove redundant MUST for minimum cgroup controllers

Any runtime which violated that constraint would necessarily violate
some more specific constraint on a 'resources' setting.

This also removes a non-spec-requirement "required" to avoid any
confusion with the spec-requirement REQUIRED [1].

[1]: opencontainers#729 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

config-linux.md

@@ -186,8 +186,7 @@ You can configure a container's cgroups via the `resources` field of the Linux c
 Do not specify `resources` unless limits have to be updated.
 For example, to run a new process in an existing container without updating limits, `resources` need not be specified.
 
-A runtime MUST at least use the minimum set of cgroup controllers required to fulfill the `resources` settings.
-However, a runtime MAY attach the container process to additional cgroup controllers supported by the system.
+Runtimes MAY attach the container process to additional cgroup controllers beyond those necessary to fulfill the `resources` settings.
 
 ###### Example