Disable TLS backend

[blurrycat]

Hello,

The library as bundled today allows choosing either OpenSSL or MbedTLS as the TLS backend for libgit2.
Would you be open to having the option of having no TLS backend at all? This would allow downstream users to reduce the size of the library.

If yes, I can cook up a PR for this :)

[Parzival-3141]

Sure. I may throw this in with a cleanup I'm planning to do, but in the meantime don't let me stop you.