Replace replace_substrs with Abseil

Bug: None
Change-Id: I155cc29db951ef1b812691c57aaafe037fbeb230
Reviewed-on: https://webrtc-review.googlesource.com/c/114241
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26699}

rtc_base/string_utils.h

@@ -93,13 +93,6 @@ inline std::string ToUtf8(const std::wstring& wstr) {
 
 #endif  // WEBRTC_WIN
 
-// Replaces all occurrences of "search" with "replace".
-void replace_substrs(const char* search,
-                     size_t search_len,
-                     const char* replace,
-                     size_t replace_len,
-                     std::string* s);
-
 // Remove leading and trailing whitespaces.
 std::string string_trim(const std::string& s);