config: Drop redundant "unique within this map" annotation requirement

This condition landed in 27a05de (Add text about extensions,
2016-06-26, opencontainers#510) with subsequent wording tweaks in 3f0440b
(config.md: add empty limit for key of annotations, Dec 28 10:35:19
2016, opencontainers#645) and 2c8feeb (config: Bring "unique... within this map"
back together, 2017-01-12, opencontainers#654).  However, since eeaccfa (glossary:
Make objects explicitly unordered and forbid duplicate names,
2016-09-27, opencontainers#584) we forbid duplicate keys on *all* objects (not just
annotations), so this PR removes the redundant annotation-specific
condition.

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

config.md

@@ -442,7 +442,6 @@ Cleanup or debugging functions are examples of such a hook.
     If there are no annotations then this property MAY either be absent or an empty map.
 
     Keys MUST be strings.
-    Keys MUST be unique within this map.
     Keys MUST NOT be an empty string.
     Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
     Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by subsequent specifications.