(reland) Retire userdata.img

it was added simply because the AS was checking for it.
With ag/4399455, the AS no longer requires it.

goog_emu_imgs no longer refers to userdata.img

Bug: 4399455
Test: delete userdata.img in an existing system image,
Test: use AS to create an AVD, boot the AVD.
Change-Id: I982edf375b6abac221796ba26e4c46a6ca8f654d
Signed-off-by: Roman Kiryanov <rkir@google.com>