[Cleanup] Add missing #include. Remove useless ones.

This CL is the result of running include-what-you-use tool on part
of the code base (audio target and dependencies) plus manual fixes.

bug: webrtc:8311
Change-Id: I277d281ce943c3ecc1bd45fd8d83055931743604
Reviewed-on: https://webrtc-review.googlesource.com/c/106280
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25311}
diff --git a/rtc_base/rtccertificate.h b/rtc_base/rtccertificate.h
index d5422f8..0adefbc 100644
--- a/rtc_base/rtccertificate.h
+++ b/rtc_base/rtccertificate.h
@@ -12,15 +12,18 @@
 #define RTC_BASE_RTCCERTIFICATE_H_
 
 #include <stdint.h>
-
 #include <memory>
+#include <string>
 
 #include "rtc_base/refcount.h"
 #include "rtc_base/scoped_ref_ptr.h"
-#include "rtc_base/sslidentity.h"
 
 namespace rtc {
 
+class SSLCertChain;
+class SSLCertificate;
+class SSLIdentity;
+
 // This class contains PEM strings of an RTCCertificate's private key and
 // certificate and acts as a text representation of RTCCertificate. Certificates
 // can be serialized and deserialized to and from this format, which allows for