ewah: drop ewah_deserialize function

ewah: drop ewah_deserialize function

We don't call this function, and in fact never have since it
was added (at least not in iterations of the ewah patches
that got merged). Instead we use ewah_read_mmap().

Let's drop the unused code.

Note to anybody who later wants to resurrect this: it does
not check for integer overflow in the ewah data size,
meaning it may be possible to convince the code to allocate
a too-small buffer and read() into it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>