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>