config: Drop "subset of the valid values" line

In 718f9f3 (minor narrative cleanup regarding config compatibility,
2017-01-30, opencontainers#673) we got:

  Implementations MUST error out when invalid values are encountered
  and MUST generate an error message and error out when encountering
  valid values it chooses to not support

In c763e64 (config: Move valid-value rules to their own section,
2017-02-07, opencontainers#681), I'd moved that out into the current "Valid values"
section with the line I'm removing in this commit.  However, giving
runtimes a blanket clause to ignore valid values makes it harder to
use runtimes, because you can't be sure an OCI-compliant runtime
supports the spec-defined value you need [1].

There have been concerns about requiring runtimes to support values
which are not supported by the host system [2].  But since 766abd6
(runtime.md: Require 'create' to fail if config.json asks for the
impossible, 2016-09-08, opencontainers#559) we've had runtime.md wording that gives
the runtime the ability to compliantly die in those cases.  That text
had a wording tweak in 72e8062 (runtime: Explicitly make process.*
timing implementation-defined, 2017-02-27, opencontainers#700), and is now:

  If the runtime cannot apply a property as specified in the
  configuration, it MUST generate an error and a new container MUST
  NOT be created.

With this commit, consumers will be able to rely on valid-value
support in compliant runtimes.  Many properties could use clearer
runtimes-MUST-support wording for those values, but we can sort those
out on a per-property basis in follow-up work.

And runtimes are still allowed to support extention values not defined
in the spec (e.g. new capability types, or mount options, or
whatever).  Like all extentions, it is up to the runtime and
runtime-caller to negotiate support in those cases.

[1]: opencontainers#813 (comment)
[2]: opencontainers#673 (comment)

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

config.md

@@ -460,7 +460,6 @@ Instead they MUST ignore unknown properties.
 ## Valid values
 
 Runtimes that are reading or processing this configuration file MUST generate an error when invalid or unsupported values are encountered.
-Unless support for a valid value is explicitly required, runtimes MAY choose which subset of the valid values it will support.
 
 ## Configuration Schema Example