Stop exporting amain for android.

[ILyoan]

rustc exports amain when build library for android to provide an start point that initiate rust runtime.
Now we have start_on_main_thread exported publicaly, a rust library is able to provide an entry function without amain hack .

#[no_mangle]
pub extern fn entry(argc: int, argv: **u8) -> int {
    do std::rt::start_on_main_thread(argc, argv) {
        // do what you want.
    }
}

Thus I don't think we need amain hack any longer.

[ILyoan]

Related to #4404

[alexcrichton]

Does this also mean that we could remove the logic of sometimes naming the entry point main and sometimes amain?

[ILyoan]

@alexcrichton Yes.
I don't think android libraries need an entry point to start runtime at all. we can remove all related code tagged as "FIXME #4404"