Remove unused methods

[ Merge of http://go/wvgerrit/16405/ ]

Methods in string conversion and license request tests were not being used.
This was detected by static analysis cppcheck reports.

b/26349052

Change-Id: I44779abf6b6fdc01b5391bff7d47be1d20ef84d2

libwvdrmengine/cdm/core/test/license_request.h

@@ -17,7 +17,6 @@ class LicenseRequest {
   ~LicenseRequest() {};
 
   void GetDrmMessage(const std::string& response, std::string& drm_msg);
-  void GetHeartbeatUrl(const std::string& response, std::string& heartbeat_url);
 
  private:
   size_t FindHeaderEndPosition(const std::string& response) const;