No longer mention APEX flattening in chroot-based testing docs.

Using flattened APEXes in ART chroot-based on-device testing is no
longer necessary.

Test: Render `README.chroot.md` as HTML and check the output manually
Bug: 121117762
Change-Id: Ic2587953acdb6ff8586d253f0a0bdf617dfff5a8
diff --git a/test/README.chroot.md b/test/README.chroot.md
index 5d03c78..b049a00 100644
--- a/test/README.chroot.md
+++ b/test/README.chroot.md
@@ -59,7 +59,6 @@
         ```
     * With a full Android (AOSP) `aosp/master` tree:
         ```bash
-        export OVERRIDE_TARGET_FLATTEN_APEX=true
         . ./build/envsetup.sh
         lunch aosp_arm64-eng  # or aosp_arm-eng for 32-bit ARM
         m adb