Remove unused icu4j prebuilt Android.bp

icu4j prebuilt is only used by ADT and layoutlib instead of our build
system.

icu4j has both prebuilt and src modules exist. When java library collect
deps, its own prebuilt dependency goes to default which is panic error.

Test: manual
Change-Id: I77ea44c911a572b6f4882fa9c51d4090c918afa7