Simplify Cygwin checks, part 1

Because OPENSSL_SYS_CYGWIN will keep OPENSSL_SYS_UNIX defined, there's
no point having checks of this form:

    #if (defined(OPENSSL_SYS_UNIX) || defined(OPENSSL_SYS_CYGWIN))
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5060)

crypto/async/arch/async_posix.h

@@ -11,7 +11,7 @@
 #define OPENSSL_ASYNC_ARCH_ASYNC_POSIX_H
 #include <openssl/e_os2.h>
 
-#if (defined(OPENSSL_SYS_UNIX) || defined(OPENSSL_SYS_CYGWIN)) \
+#if defined(OPENSSL_SYS_UNIX) \
     && defined(OPENSSL_THREADS) && !defined(OPENSSL_NO_ASYNC) \
     && !defined(__ANDROID__) && !defined(__OpenBSD__)
 

engines/e_dasync.c

@@ -25,7 +25,7 @@
 #include <openssl/ssl.h>
 #include <openssl/modes.h>
 
-#if (defined(OPENSSL_SYS_UNIX) || defined(OPENSSL_SYS_CYGWIN)) && defined(OPENSSL_THREADS)
+#if defined(OPENSSL_SYS_UNIX) && defined(OPENSSL_THREADS)
 # undef ASYNC_POSIX
 # define ASYNC_POSIX
 # include <unistd.h>