[XrdTls] Use TLS_method() unconditionally

This function is always available with OpenSSL >= 1.0.2 (required for building
XRootD). The man page for TLS_method(3) also says:

TLS_method(), TLS_server_method(), TLS_client_method()
  These are the general-purpose version-flexible SSL/TLS methods.  The actual
  protocol version used will be negotiated to the highest version mutually
  supported by the client and the server.  The supported protocols are SSLv3,
  TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3.  Applications should use these methods,
  and avoid the version-specific methods described below, which are deprecated.

SSLv23_method(), SSLv23_server_method(), SSLv23_client_method()
  These functions do not exist anymore, they have been renamed to TLS_method(),
  TLS_server_method() and TLS_client_method() respectively.  Currently, the old
  function calls are renamed to the corresponding new ones by preprocessor
  macros, to ensure that existing code which uses the old function names still
  compiles. However, using the old function names is deprecated and new code
  should call the new functions instead.

src/XrdTls/XrdTlsContext.cc

@@ -429,11 +429,7 @@ void Fatal(std::string *eMsg, const char *msg, bool sslmsg=false)
 
 const char *GetTlsMethod(const SSL_METHOD *&meth)
 {
-#ifdef HAVE_TLS
   meth = TLS_method();
-#else
-  meth = SSLv23_method();
-#endif
   if (meth == 0) return "No negotiable TLS method available.";
   return 0;
 }