webauthn: remove explicit color for caBLEv2 serverlink.

This color reflected the current practice in Play Services, but doesn't
look good in dark mode. The default color works well for text and
changes automatically to match the theme.

BUG=1002262

Change-Id: Ib9d852f17eeec096ea4e11ce670782b2ecc6d702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596111
Commit-Queue: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#837727}

chrome/android/features/cablev2_authenticator/java/res/layout/cablev2_serverlink.xml

@@ -64,6 +64,5 @@
         a:layout_marginTop="4dp"
         a:layout_gravity="center_horizontal"
         a:padding="0px"
-        a:textColor="@color/modern_grey_800"
         a:textSize="14sp"/>
 </LinearLayout>