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.h

@@ -37,6 +37,14 @@
 
 namespace rtc {
 
+namespace webrtc_openssl_adapter_internal {
+
+// Local definition, since absl::StrJoin is not allow-listed. Declared in header
+// file only for unittests.
+std::string StrJoin(const std::vector<std::string>& list, char delimiter);
+
+}  // namespace webrtc_openssl_adapter_internal
+
 class OpenSSLAdapter final : public SSLAdapter,
                              public MessageHandlerAutoCleanup {
  public: