Revert "Added Windows Cryptography API: Next Generation based backend"

This reverts commit d385230e15715e67796f16f3e65fd899f21a638b.
configure.ac: Add zlib to Requires.private in libssh2.pc if using zlib
Clean up crypto library abstraction in build system and source code

libssh2 used to explicitly check for libgcrypt and default to OpenSSL.

Now all possible crypto libraries are checked for explicitly, making
the addition of further crypto libraries both simpler and cleaner.
configure.ac: Rework crypto library detection

This further simplifies adding new crypto libraries.
configure.ac: Reorder --with-* options in --help output
configure.ac: Call zlib zlib and not libz in text but keep option names