Delete rtc::string_trim. Replaced with absl::StripAsciiWhitespace.

Bug: webrtc:6424, webrtc:13579
Change-Id: I222e1bfb62d5f1f1a2c74e5fce1038e04e7bebfb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/255824
Reviewed-by: Ali Tofigh <alito@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36234}

rtc_base/string_utils.h

@@ -82,9 +82,6 @@ inline std::string ToUtf8(const std::wstring& wstr) {
 
 #endif  // WEBRTC_WIN
 
-// Remove leading and trailing whitespaces.
-std::string string_trim(const std::string& s);
-
 // TODO(jonasolsson): replace with absl::Hex when that becomes available.
 std::string ToHex(int i);