Stop defining _WINDLL

It doesn't seem necessary. If it _is_ necessary in some build
config, we can add it back with a comment saying what it's for.

Change-Id: If9321106ee76428f8702464820eb34790157ab56
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55689
Reviewed-by: Nico Weber <thakis@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>
Tested-by: Nico Weber <thakis@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 769fddc..9b56c6d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -24,7 +24,6 @@
 
   if (is_win) {
     defines += [
-      "_WINDLL",
       "NO_SANITIZE_FUNCTION=",
 
       # Disable MSVC warnings about std::aligned_storage being broken before