Remove references to the pdk product variable

pdk is deprecated and has no effect as of aosp/1319667.

Test: Presubmits
Change-Id: I5ac91622516fc434c52f3d8d2aa4a37639dda572
diff --git a/Android.bp b/Android.bp
index 2bff72c..48a4bc3 100644
--- a/Android.bp
+++ b/Android.bp
@@ -75,12 +75,6 @@
     ],
     ldflags: ["-Wl,--gc-sections"],
 
-    product_variables: {
-        pdk: {
-            enabled: false,
-        },
-    },
-
     target: {
         android: {
             cflags: [