Clean up SimpleStringBuilder after absl::string_view adoption

Bug: webrtc:13579
Change-Id: I8fdc65e19893c9a96b263bbc99c4f9139fa4b78f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/256268
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Ali Tofigh <alito@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36291}

rtc_base/strings/string_builder.h

@@ -60,10 +60,6 @@ class SimpleStringBuilder {
   SimpleStringBuilder&
   AppendFormat(const char* fmt, ...);
 
-  // An alternate way from operator<<() to append a string. This variant is
-  // slightly more efficient when the length of the string to append, is known.
-  SimpleStringBuilder& Append(const char* str, size_t length);
-
  private:
   bool IsConsistent() const {
     return size_ <= buffer_.size() - 1 && buffer_[size_] == '\0';