Revert "Try /MTd on Debug Windows builds."

This reverts commit 58ce2bc51a318e53e3e6f1ef297c5adc775b0fec.

Reason for revert: warnings / errors when building ANGLE.

Original change's description:
> Try /MTd on Debug Windows builds.
> 
> The people clamor for it!  I doubt we'll care in our testing.
> 
> BUG=skia:5928
> 
> Change-Id: I80d3948fd1f5d0b956c308bc3a6183b8660575ae
> Reviewed-on: https://skia-review.googlesource.com/7364
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> Commit-Queue: Ben Wagner <bungeman@google.com>
> Reviewed-by: Ben Wagner <bungeman@google.com>
> 

TBR=mtklein@chromium.org,bungeman@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:5928

Change-Id: Id6ab3de7ebb5394749233429bada2f5244ff2978
Reviewed-on: https://skia-review.googlesource.com/7372
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 58cfb23..23e5288 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -132,9 +132,7 @@
   "//gn:warnings",
   "//gn:warnings_except_public_headers",
 ]
-if (is_debug) {
-  default_configs += [ "//gn:debug" ]
-} else {
+if (!is_debug) {
   default_configs += [ "//gn:release" ]
 }
 if (!is_official_build) {