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_fuchsia_device.go b/cc/config/x86_64_fuchsia_device.go
index 79af00c..0f2013b 100644
--- a/cc/config/x86_64_fuchsia_device.go
+++ b/cc/config/x86_64_fuchsia_device.go
@@ -92,7 +92,7 @@
 }
 
 func (t *toolchainFuchsiaX8664) ToolchainClangCflags() string {
-	return "-DUSE_SSSE3 -mssse3"
+	return "-mssse3"
 }
 
 var toolchainFuchsiaSingleton Toolchain = &toolchainFuchsiaX8664{}