Revert "Enable iOS Captive Portal feature by default."

This reverts commit beecf799.

Reason for revert:

The following tests from ios_internal_chrome_external_url_egtests are broken:
ExternalURLSSLTestCase/testReloadInterstitial
ExternalURLSSLTestCase/testGoBackFromInterstitialUsingInterstitialButton
ExternalURLSSLTestCase/testLoadNTPAfterDontProceed
ExternalURLSSLTestCase/testAcceptingBadSSLCert
ExternalURLSSLTestCase/testRememberingAcceptUserDecision
ExternalURLSSLTestCase/testGoBackFromInterstitialUsingToolbarButton
ExternalURLSSLTestCase/testHostnameMismatchInterstitial

stdout from bot run here: https://logs.chromium.org/v/?s=chrome%2Fbb%2Finternal.bling.main%2Fiphone11-simulator-x64%2F1975%2F%2B%2Frecipes%2Fsteps%2Fios_internal_chrome_external_url_egtests__iPhone_5s_iOS_11.0__on_Mac%2F0%2Fstdout

Original change's description:
> Enable iOS Captive Portal feature by default.
> 
> Bug: 750228
> Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
> Change-Id: I0fa138e855d71ac256759d75f3e84d390cd2c896
> Reviewed-on: https://chromium-review.googlesource.com/777468
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#517579}

TBR=eugenebut@chromium.org,michaeldo@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 750228
Change-Id: Ic15218338b16eede6572cbe157d982942de0a625
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/778283Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517727}

ios/chrome/browser/ssl/captive_portal_features.cc

@@ -5,4 +5,4 @@
 #include "ios/chrome/browser/ssl/captive_portal_features.h"
 
 const base::Feature kCaptivePortalFeature{"CaptivePortalFeature",
-                                          base::FEATURE_ENABLED_BY_DEFAULT};
+                                          base::FEATURE_DISABLED_BY_DEFAULT};