Revert "Use /MD for Windows builds."

This reverts commit cab79aadad.

Reason for revert: Bots crashing.

Original change's description:
> Use /MD for Windows builds.
> 
> I think the default is /MT, static linking.
> This should make our builds smaller, and compatible with clients using /MD.
> 
> Change-Id: Id8a39a029925eda2627532bbd0223c693300f5da
> Reviewed-on: https://skia-review.googlesource.com/5277
> Reviewed-by: Ben Wagner <bungeman@google.com>
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> 

TBR=mtklein@chromium.org,bungeman@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: Iff644250a23175e44a4282e7aaea0e2a2adc1ce0
Reviewed-on: https://skia-review.googlesource.com/5310
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Joe Gregorio <jcgregorio@google.com>

gn/BUILD.gn

@@ -64,7 +64,6 @@ config("default") {
       "/FS",  # Preserve previous PDB behavior.
       "/bigobj",  # Some of our files are bigger than the regular limits.
       "/WX",  # Treat warnings as errors.
-      "/MD",  # Link against dynamic, non-debug standard library.
     ]
     defines += [
       "_CRT_SECURE_NO_WARNINGS",  # Disables warnings about sscanf().