[Signin][Android] Remove SigninTest file after refactoring

This CL removes the SigninTest since it is all refactored and
reimplemented in SigninSignoutIntegrationTest.

Bug: 1038326
Change-Id: I56c0097be1ffd10feea7d5fec9b30d00281730bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056843Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742061}

chrome/android/chrome_test_java_sources.gni

@@ -446,7 +446,6 @@ chrome_test_java_sources = [
   "javatests/src/org/chromium/chrome/browser/signin/SigninFragmentTest.java",
   "javatests/src/org/chromium/chrome/browser/signin/SigninHelperTest.java",
   "javatests/src/org/chromium/chrome/browser/signin/SigninSignoutIntegrationTest.java",
-  "javatests/src/org/chromium/chrome/browser/signin/SigninTest.java",
   "javatests/src/org/chromium/chrome/browser/sms/SmsReceiverInfoBarTest.java",
   "javatests/src/org/chromium/chrome/browser/ssl/CaptivePortalTest.java",
   "javatests/src/org/chromium/chrome/browser/status_indicator/StatusIndicatorTest.java",