Drop defunct Windows compat fixes

"The AI_ADDRCONFIG flag is defined on the Windows SDK for Windows Vista
and later. The AI_ADDRCONFIG flag is supported on Windows Vista and
later."
https://docs.microsoft.com/en-us/windows/desktop/api/ws2tcpip/nf-ws2tcpip-getaddrinfo

However, the version of MinGW we use on Travis is not current and does
not carry the relevant definition, as such I defined it in compat.
https://github.com/wine-mirror/wine/blob/master/include/ws2tcpip.h

Testing confirms that the PROTECTION_LEVEL_UNRESTRICTED,
IPV6_PROTECTION_LEVEL, PROCESS_DEP_ENABLE, AI_ADDRCONFIG, are now
supported by the version of Windows that we test against, so can be
removed.
https://travis-ci.org/bitcoin/bitcoin/jobs/483255439
https://travis-ci.org/Empact/bitcoin/jobs/484123087

src/net.cpp

@@ -58,17 +58,6 @@ static constexpr int DUMP_PEERS_INTERVAL = 15 * 60;
 #define MSG_DONTWAIT 0
 #endif
 
-// Fix for ancient MinGW versions, that don't have defined these in ws2tcpip.h.
-// Todo: Can be removed when our pull-tester is upgraded to a modern MinGW version.
-#ifdef WIN32
-#ifndef PROTECTION_LEVEL_UNRESTRICTED
-#define PROTECTION_LEVEL_UNRESTRICTED 10
-#endif
-#ifndef IPV6_PROTECTION_LEVEL
-#define IPV6_PROTECTION_LEVEL 23
-#endif
-#endif
-
 /** Used to pass flags to the Bind() function */
 enum BindFlags {
     BF_NONE         = 0,

src/netbase.cpp

@@ -80,11 +80,7 @@ bool static LookupIntern(const char *pszName, std::vector<CNetAddr>& vIP, unsign
     aiHint.ai_socktype = SOCK_STREAM;
     aiHint.ai_protocol = IPPROTO_TCP;
     aiHint.ai_family = AF_UNSPEC;
-#ifdef WIN32
-    aiHint.ai_flags = fAllowLookup ? 0 : AI_NUMERICHOST;
-#else
     aiHint.ai_flags = fAllowLookup ? AI_ADDRCONFIG : AI_NUMERICHOST;
-#endif
     struct addrinfo *aiRes = nullptr;
     int nErr = getaddrinfo(pszName, nullptr, &aiHint, &aiRes);
     if (nErr)