Remove no-longer-used autofill card unmask strings from grit whitelist.

The usage of these strings was removed in
https://codereview.chromium.org/1226933004/.

BUG=484806

Review URL: https://codereview.chromium.org/1228343003

Cr-Original-Commit-Position: refs/heads/master@{#338382}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 85736428d58d053ced58a706c56dcc6cecca9db9

ios/grit_whitelist.txt

@@ -143,8 +143,6 @@ IDS_AUTOFILL_CARD_UNMASK_PROMPT_INSTRUCTIONS
 IDS_AUTOFILL_CARD_UNMASK_PROMPT_INSTRUCTIONS_AMEX
 IDS_AUTOFILL_CARD_UNMASK_PROMPT_INSTRUCTIONS_EXPIRED
 IDS_AUTOFILL_CARD_UNMASK_PROMPT_INSTRUCTIONS_EXPIRED_AMEX
-IDS_AUTOFILL_CARD_UNMASK_PROMPT_TITLE
-IDS_AUTOFILL_CARD_UNMASK_PROMPT_UPDATE_TITLE
 IDS_AUTOFILL_CC_AMEX
 IDS_AUTOFILL_CC_AMEX_SHORT
 IDS_AUTOFILL_CC_DINERS