remove rpm spec file and readme

Fedora provides a rpm package for libgit2 - https://apps.fedoraproject.org/packages/libgit2

The bundled libgit2 spec is extremely outdated (v0.16), and it's broken for the latest libgit2 release. If someone needs a libgit2 rpm for Suse, CentOS or RHEL , the best way is to go trough the package submission process of the target distro. 

For custom builds the Fedora RPM spec provides a great starting point and it will be kept up to date with libgit2 releases.