Remove unnecessary use of "explicit"

It's not used anywhere else with constructors taking more than one
(non-optional) argument and is not really useful with them (and not
useful at all until C++17), so don't use it here neither for
consistency.

No real changes.

include/wx/webrequest_curl.h

@@ -23,7 +23,7 @@ class wxWebRequestCURL;
 class WXDLLIMPEXP_NET wxWebAuthChallengeCURL : public wxWebAuthChallenge
 {
 public:
-    explicit wxWebAuthChallengeCURL(Source source, wxWebRequestCURL& request);
+    wxWebAuthChallengeCURL(Source source, wxWebRequestCURL& request);
 
     bool Init();