Delete rtc_base/unittest_main.cc

Usage replaced with test/test_main.cc.

Bug: webrtc:5996
Change-Id: I65e7539f2072fb45255a3c1af0b10dd06e1701ca
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137805
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28010}

rtc_base/experiments/BUILD.gn

@@ -204,12 +204,13 @@ if (rtc_include_tests) {
       ":rate_control_settings",
       ":rtt_mult_experiment",
       "..:gunit_helpers",
-      "../:rtc_base_tests_main",
       "../:rtc_base_tests_utils",
       "../../api/video_codecs:video_codecs_api",
       "../../system_wrappers:field_trial",
       "../../test:field_trial",
+      "../../test:test_main",
       "../../test:test_support",
+      "//third_party/abseil-cpp/absl/types:optional",
     ]
   }
 }