[iOS] Cleanup no-op formatting text in omnibox.

Historically, omnibox was showing colorized URLs (scheme in green or
red, path in lighter gray etc). With today's iOS location bar, the
textfield only ever shows unattributed text. Cleanup this unused logic:

OmniboxViewIOS::UpdateTextStyle() is no-op because it only calls
SetEmphasis and UpdateSchemeStyle, which are both no-ops in the iOS
implementation.

This means that ApplyTextAttributes() is simply converting a UTF16
string to NSAttributedString. Since it's an iOS-only method, replace
it with [[NSMutableAttributedString alloc]
 initWithString:base::SysUTF16ToNSString(text)] calls.

Now attributing_display_string_ is unused. Remove it. Also remove
unused GetSecureTextColor() method and its helpers.

Bug: none
Change-Id: I9470d461a3d658e297c0e2f818010f338a01de04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3431527
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#966203}