Remove the Legacy Symantec PKI policy

Remove the Enterprise Policy for enabling the Legacy Symantec
PKI. Certificates issued directly by the legacy PKI will no longer
be trusted by default in any version of Chrome.

Support for the SSLConfig option is still maintained for embedders,
such as Android WebView, which may wish to adhere to locally defined
or system-wide policies around trust.

Bug: 953321
Change-Id: I15cb23734be19e3ec53af0e9adcc49102fe06550
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1569515
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651714}