config: Remove "MAY support any valid values" sentence

This line landed in 718f9f3 (origin/pr/673) minor narrative cleanup
regarding config compatibility, 2017-01-30, opencontainers#673), but I'm not clear
on the motivation.  The wording reads to me like "you don't have to
support valid values for platform-specific fields if you don't want
to", but we already cover unsupported value handling with the "MUST
generate an error when invalid or unsupported values are encountered"
language separated out in c763e64 (config: Move valid-value rules to
their own section, 2017-02-07, opencontainers#681).  This commit removes the
ambiguous line.

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

Author: wking

config.md

@@ -340,7 +340,6 @@ For Windows based systems the user structure has the following fields:
 ## <a name="configPlatformSpecificConfiguration" />Platform-specific configuration
 
 [**`platform.os`**](#platform) is used to specify platform-specific configuration.
-    Runtime implementations MAY support any valid values for platform-specific fields as part of this configuration.
 
 * **`linux`** (object, OPTIONAL) [Linux-specific configuration](config-linux.md).
     This MAY be set if **`platform.os`** is `linux` and MUST NOT be set otherwise.