We haven't required a local copy of the ca-certificates since switching
to OpenSSL as the backend for TLS. Remove the script to download the
PEM file, and update the tests to use the system's CA certificates.
By moving `Certificate` to `LibCrypto` it is possible to reuse a bunch
of code from in `LibCrypto` itself. It also moves some constants
and pieces of code to a more appropriate place than `LibTLS`.
This also makes future work on WebCryptoAPI easier.
The declaration of `DefaultRootCACertificates` was in `Certificate.h`
and its implementation in `TLSv12.cpp`. It has been moved over
to `TLSv12.h` for consistency.
This is in preparation of the next commits to split the changes.
