hash: use CommonCrypto on OSX for SHA-1

OSX has its own cryptographic library, let's make use of it instead of
calling out to OpenSSL.
remote: delete git_remote_supported_url()

This function does not in fact tell us anything, as almost anything with
a colon in it is a valid rsync-style SSH path; it can not tell us that
we do not support ftp or afp or similar as those are still valid SSH
paths and we do support that.
Merge pull request #2644 from libgit2/cmn/remove-supported-url

remote: delete git_remote_supported_url()
Merge pull request #2645 from libgit2/cmn/common-crypto

[RFC] Use CommonCrypto for hashing