Revert "Fix "Add skip_themes directive to build process""

This reverts commit 630196ebe21ff308011c64f8ac491a718607623b.
diff --git a/products/themes.mk b/products/themes.mk
index 856ab1f..00d1045 100644
--- a/products/themes.mk
+++ b/products/themes.mk
@@ -1,7 +1,7 @@
 # Theme packages
 
-ifneq ($(SKIP_THEMES),true)
+ifndef SKIP_THEMES
 PRODUCT_PACKAGES += \
        Androidian \
        Cyanbread
-endif
+endif
\ No newline at end of file