Remove references to the pdk product variable

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

Test: Presubmits
Change-Id: I78d7afaaa71b93e575dcd5c20716ffb295b9f522
diff --git a/bootstat/Android.bp b/bootstat/Android.bp
index ca59ef3..0c8760c 100644
--- a/bootstat/Android.bp
+++ b/bootstat/Android.bp
@@ -72,9 +72,6 @@
     ],
     init_rc: ["bootstat.rc"],
     product_variables: {
-        pdk: {
-            enabled: false,
-        },
         debuggable: {
             init_rc: ["bootstat-debug.rc"],
         },