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
1 file changed