cygwin: Remove the CYGWIN_V15_WIN32API build variable

cygwin: Remove the CYGWIN_V15_WIN32API build variable

Commit 380a4d92 ("Update cygwin.c for new mingw-64 win32 api headers",
11-11-2012) solved an header include order problem on cygwin 1.7 when
using the new mingw-64 WIN32 API headers. The solution involved using
a new build variable (V15_MINGW_HEADERS) to conditionally compile the
cygwin.c source file to use an include order appropriate for the old
and new header files. (The build variable was later renamed in commit
9fca6cff to CYGWIN_V15_WIN32API).

The include order used for cygwin 1.7 includes the "win32.h" header
before "../git-compat-util.h". This order was problematic on cygwin
1.5, since it lead to the WIN32 symbol being defined along with the
inclusion of some WIN32 API headers (e.g. <winsock2.h>) which cause
compilation errors.

The header include order problem on cygwin 1.5 has since been fixed
(see commit "mingw: rename WIN32 cpp macro to GIT_WINDOWS_NATIVE"),
so we can now remove the conditional compilation along with the
associated CYGWIN_V15_WIN32API build variable.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>