config-linux: Remove explicit 'null' from device cgroup values

Catch the Markdown spec up with the JSON Schema change in 0927437
(schema: Drop pointers and nulls, 2017-01-18, opencontainers#662).  The Markdown is
canonical, so we could restore the explicit-null handling to the JSON
Schema instead, but the maintainers feel (and I agree) that there's no
point in explicitly allowing a null value when callers can simply
leave the property unset [1].

[1]: opencontainers#555 (comment)

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

config-linux.md

@@ -218,9 +218,9 @@ Each entry has the following structure:
 
 * **`allow`** *(boolean, REQUIRED)* - whether the entry is allowed or denied.
 * **`type`** *(string, OPTIONAL)* - type of device: `a` (all), `c` (char), or `b` (block).
-  `null` or unset values mean "all", mapping to `a`.
+  Unset values mean "all", mapping to `a`.
 * **`major, minor`** *(int64, OPTIONAL)* - [major, minor numbers][devices] for the device.
-  `null` or unset values mean "all", mapping to [`*` in the filesystem API][cgroup-v1-devices].
+  Unset values mean "all", mapping to [`*` in the filesystem API][cgroup-v1-devices].
 * **`access`** *(string, OPTIONAL)* - cgroup permissions for device.
   A composition of `r` (read), `w` (write), and `m` (mknod).