Revert "Update libz to VNDK-SP in generic system image"

This reverts commit 06eb4036925ac4634fb9a2a85231824718d1532c.  This
commit removes libz.so from /system/lib[64]/vndk-sp.

We have to remove libz.so from vndk-sp because we don't have a good
mechanism to stop "classloader" linker namespace from loading
/system/lib[64]/vndk-sp/libz.so.  Other alternatives require significant
changes to dynamic linker or CTS test cases.

Let's temporarily revert the commit to fix b/62195853 for oc-dr1-dev.
The long-term solution will be discussed in b/37617391.

Bug: 62195853
Bug: 37617391
Test: Boot sailfish + GSI and see the home screen
Change-Id: I0d94bbf38d91c068e301c969e9615135124ebb1d
diff --git a/vndk/vndk-sp-libs.mk b/vndk/vndk-sp-libs.mk
index e6034f3..2e1c2bb 100644
--- a/vndk/vndk-sp-libs.mk
+++ b/vndk/vndk-sp-libs.mk
@@ -25,4 +25,3 @@
     libpng \
     libunwind \
     libutils \
-    libz \