Remove remnants of DHE support.

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below. The name of the top level entry should be of the form <policy name>[.suffix]. The optional suffix is used for defining multiple test cases for a single policy.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "android", "Defaults to empty if not specified."],
     "official_only": "Whether this policy exists in official builds only. Defaults to |false| if not specified.",
     "can_be_recommended": "Whether a recommended value may be set for the policy. Defaults to |false| if not specified.",
     "test_policy": "A policy dictionary that should make the preferences affected by this policy become policy-controlled. Usually just sets the current policy. Defaults to an empty dictionary if not specified.",
     "note": "If the policy affects any preferences, the following array should be specified with one entry per such preference.",
     "pref_mappings": [
@@ skipping 1943 matching lines @@
     "test_policy": { "CertificateTransparencyEnforcementDisabledForUrls": ["example.com"] },
     "pref_mappings": [
       { "pref": "certificate_transparency.excluded_hosts" }
     ]
   },
 
   "RC4Enabled": {
   },
 
   "DHEEnabled": {
-    "os": ["win", "linux", "mac", "chromeos"],

[pastarmovj, 2017/01/23 23:28:06]

Please use a "note" field to mark this policy as deprecated. See line 384 for an
example.

[davidben, 2017/01/24 21:00:03]

Done. Also added it for the others I'd previously emptied.

-    "test_policy": { "DHEEnabled": true },
-    "pref_mappings": [
-      { "pref": "ssl.dhe_enabled",
-        "local_state": true
-      }
-    ]
   },
 
   "WelcomePageOnOSUpgradeEnabled": {
     "os": ["win"],
     "test_policy": { "WelcomePageOnOSUpgradeEnabled": false },
     "pref_mappings": [
       { "pref": "browser.welcome_page_on_os_upgrade_enabled",
         "local_state": true
       }
     ]
@@ skipping 1063 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }