Change fill-on-account-select to use Features API, not a custom flag

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 9a8f1ece0dc4bb9cf2aed4aa80f6a9ecc2ef0623..fb83920c44ec7efcee23449e718a51a40f0b1446 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -7430,12 +7430,6 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FILL_ON_ACCOUNT_SELECT_DESCRIPTION" desc="Description of the experiment for the password manager to fill on account selection rather than page load">
         Filling of passwords when an account is explicitly selected by the user rather than autofilling credentials on page load.
       </message>
-      <message name="IDS_FLAGS_FILL_ON_ACCOUNT_SELECT_ENABLE_HIGHLIGHTING" desc="Option to enable fill on account select with field highlighting">
-        Enable with field highlighting
-      </message>
-      <message name="IDS_FLAGS_FILL_ON_ACCOUNT_SELECT_ENABLE_NO_HIGHLIGHTING" desc="Option to enable fill on account select with no field highlighting">
-        Enable with no field highlighting
-      </message>
       <if expr="not is_android">
         <message name="IDS_PASSWORD_MANAGER_SAVE_BUTTON" desc="Save button text for password manager">
           Save