Remove content-less string IDS_ERRORPAGES_ERROR_CODE.

This string contains a placeholder and no actual text. It doesn't need
to exist.

BUG=518020

Review URL: https://codereview.chromium.org/1339653005

Cr-Original-Commit-Position: refs/heads/master@{#348824}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c3950164bfb0278be2818abc7229ecbd4318f9a2

ios/grit_whitelist.txt

@@ -391,7 +391,6 @@ IDS_ERRORPAGES_DETAILS_TEMPORARILY_THROTTLED
 IDS_ERRORPAGES_DETAILS_TIMED_OUT
 IDS_ERRORPAGES_DETAILS_TOO_MANY_REDIRECTS
 IDS_ERRORPAGES_DETAILS_UNKNOWN
-IDS_ERRORPAGES_ERROR_CODE
 IDS_ERRORPAGES_HEADING_ACCESS_DENIED
 IDS_ERRORPAGES_HEADING_BAD_SSL_CLIENT_AUTH_CERT
 IDS_ERRORPAGES_HEADING_BLOCKED