remove wrapper code for Windows CryptoAPI function

remove wrapper code for Windows CryptoAPI function

The CryptoAPI function CryptAcquireCertificatePrivateKey() was previously
unsupported in MinGW. With recent w32api headers it's now defined, mostly.
Since the code used to load the CryptoAPI DLL is prone to a DLL hijacking
attack [1], it's now a good time to get rid of wrapper completely.

Just a few macros left that may still be undefined on some
systems using the original MinGW headers.

[1] http://isc.sans.edu/diary.html?storyid=9445

Signed-off-by: Heiko Hund <heiko.hund@sophos.com>
Acked-by: James Yonan <james@openvpn.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/4979
Signed-off-by: David Sommerseth <davids@redhat.com>