Revert "Weak-link against libssh2."
Revert "Weak-link against libssh2."

This reverts commit 06d171dc43b8aae2967c0d1e8767da79acd88309.

@tiennou This is failing on my machine and the build server, due to libssh2 not being found. It sounds like the weak option isn't working correctly?

misterfifths/objective-git,libgit2/objective-git,dleehr/objective-git,tiennou/objective-git,c9s/objective-git,tiennou/objective-git,Acidburn0zzz/objective-git,javiertoledo/objective-git,phatblat/objective-git,blackpixel/objective-git,slavikus/objective-git,0x4a616e/objective-git,pietbrauer/objective-git,misterfifths/objective-git,TOMalley104/objective-git,slavikus/objective-git,misterfifths/objective-git,nerdishbynature/objective-git,libgit2/objective-git,0x4a616e/objective-git,TOMalley104/objective-git,0x4a616e/objective-git,slavikus/objective-git,Acidburn0zzz/objective-git,pietbrauer/objective-git,nerdishbynature/objective-git,javiertoledo/objective-git,nerdishbynature/objective-git,blackpixel/objective-git,Acidburn0zzz/objective-git,pietbrauer/objective-git,c9s/objective-git,TOMalley104/objective-git,Acidburn0zzz/objective-git,phatblat/objective-git,dleehr/objective-git,tiennou/objective-git,javiertoledo/objective-git,pietbrauer/objective-git,TOMalley104/objective-git,c9s/objective-git,misterfifths/objective-git,c9s/objective-git,libgit2/objective-git,blackpixel/objective-git,dleehr/objective-git,alehed/objective-git,alehed/objective-git,phatblat/objective-git,dleehr/objective-git,alehed/objective-git,blackpixel/objective-git,javiertoledo/objective-git,libgit2/objective-git