Delete rtc::join

Define a local function in the only place where it is used, for calling
SSL_set1_curves_list.

Bug: webrtc:6424
Change-Id: I7b9c372aaed15dbc88ced55652f5afd93db55e49
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261313
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36853}

rtc_base/openssl_adapter_unittest.cc

@@ -113,4 +113,19 @@ TEST(OpenSSLAdapterFactoryTest, CreateWorksWithCustomVerifier) {
   EXPECT_NE(simple_adapter, nullptr);
 }
 
+TEST(StrJoinTest, SingleElement) {
+  EXPECT_EQ(webrtc_openssl_adapter_internal::StrJoin({"a"}, ','), "a");
+}
+
+TEST(StrJoinTest, TwoElements) {
+  EXPECT_EQ(webrtc_openssl_adapter_internal::StrJoin({"first", "second"}, ':'),
+            "first:second");
+}
+
+TEST(StrJoinTest, WithEmptyElement) {
+  EXPECT_EQ(
+      webrtc_openssl_adapter_internal::StrJoin({"first", "", "second"}, ':'),
+      "first::second");
+}
+
 }  // namespace rtc