Remove unused variables in Windows code

Upstreaming internal changes required to enforce `-Wunused-variable`
and `-Wunused-result` on Windows.

Tested: ci
Change-Id: I98d9ba8eeebf4dd8520b4263f4a57039c28020ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404840
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Auto-Submit: Ivan Inozemtsev <iinozemtsev@google.com>

runtime/bin/eventhandler_win.cc

@@ -852,7 +852,7 @@ intptr_t ClientSocket::disconnecting_ = 0;
 #endif
 
 void ClientSocket::Shutdown(int how) {
-  int rc = shutdown(socket(), how);
+  shutdown(socket(), how);
   if (how == SD_RECEIVE) {
     MarkClosedRead();
   }

runtime/vm/native_symbol_win.cc

@@ -63,7 +63,6 @@ const char* NativeSymbolResolver::LookupSymbolName(uword pc, uword* start) {
   const intptr_t kMaxNameLength = 2048;
   const intptr_t kSymbolInfoSize = sizeof(SYMBOL_INFO);  // NOLINT.
   static char buffer[kSymbolInfoSize + kMaxNameLength];
-  static char name_buffer[kMaxNameLength];
   MutexLocker lock(lock_);
   if (!running_) {
     return nullptr;

runtime/vm/os_win.cc

@@ -31,7 +31,6 @@ intptr_t OS::ProcessId() {
 const int64_t kTimeEpoc = 116444736000000000LL;
 
 static bool LocalTime(int64_t seconds_since_epoch, tm* tm_result) {
-  time_t seconds = static_cast<time_t>(seconds_since_epoch);
   SYSTEMTIME systemTime;
   union {
     FILETIME fileTime;