cmake: remove (_)UNICODE def on Windows in CMakeLists.txt

cmake: remove (_)UNICODE def on Windows in CMakeLists.txt

`UNICODE` and `_UNICODE` are not required when building git on Windows.
Actually, they should not be predefined at all.

There're 2 evidences that `(_)UNICODE` is supposed to be nonexist:

compat/win32/trace2_win32_process_info.c:83: It uses jw_array_string
which accepts pe32.szExeFile as const char*.

t/helper/test-drop-caches.c:16: Calling to GetCurrentDirectory with
Buffer as char*.

The autotools build system never defines `UNICODE` and `_UNICODE` and
builds on Windows well.

Signed-off-by: Yuyi Wang <Strawberry_Str@hotmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>