Stop defining unused USE_SSSE3.

This copy & paste has even made it into the fuchsia configuration...

Test: builds
Change-Id: If22615fdf04448c058db014d93155e6264fb3eb2
diff --git a/cc/config/x86_64_device.go b/cc/config/x86_64_device.go
index d9c96df..bcfae5d 100644
--- a/cc/config/x86_64_device.go
+++ b/cc/config/x86_64_device.go
@@ -63,7 +63,7 @@
 	}
 
 	x86_64ArchFeatureCflags = map[string][]string{
-		"ssse3":  []string{"-DUSE_SSSE3", "-mssse3"},
+		"ssse3":  []string{"-mssse3"},
 		"sse4":   []string{"-msse4"},
 		"sse4_1": []string{"-msse4.1"},
 		"sse4_2": []string{"-msse4.2"},