Remove AX_CHECK_OPENSSL macro

OpenSSL supports pkg-config method since the 0.9.8 version and we already require pkg-config for other mandatory libraries. Also the way the AX_CHECK_OPENSSL macro was integrated into the configure script was confusing - the macro would be used only if the libcrypto.pc and libssl.pc file are not usable, so calling ./configure --with-openssl=/usr/local would have no effect when PKG_CHECK_MODULES would be successful.

Closes #2806 (closed)