Delete left-over declaration of AdjustCurrentProcessPrivilege.

Implementation was deleted in https://codereview.webrtc.org/2571763002.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2872303002
Cr-Commit-Position: refs/heads/master@{#18082}

webrtc/base/win32.h

@@ -122,8 +122,6 @@ inline bool IsCurrentProcessLowIntegrity() {
       level < SECURITY_MANDATORY_MEDIUM_RID);
 }
 
-bool AdjustCurrentProcessPrivilege(const TCHAR* privilege, bool to_enable);
-
 }  // namespace rtc
 
 #endif  // WEBRTC_WIN