Fix build warnings for CaptivePortalLogin

CaptivePortalLogin needs only the defintion in
metrics_constants.proto. This will not need the jarjar_rules.
Thus, remove the unnecessary rule to fix the build warning.

Test: make CaptivePortalLogin and no warning
Bug: 123717273
Change-Id: I5bb3ee61be769989d02a2762a7747d4dafd7bd71