um: remove unused mm_copy_segments

It was already removed by commit c17c0204 ("arch: remove unused
*_segments() macros/functions") but seems to have been accidentally
reintroduced by commit 0500871f ("Construct init thread stack in the
linker script rather than by union"). Remove it for good.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Richard Weinberger <richard@nod.at>