Remote property accessor on branch branch should not throw an exception

When a branch is under the refs/remotes/ namespace, but a specific remote cannot be resolved,
then the Remote property accessor should return null instead of throwing an exception.
It looks like it's been bring in through https://github.com/libgit2/libgit2sharp/commit/4e437239db67d3f4e3f2a53554a1768122a07300. But I don not remember why it's been added. Let's get rid of it provided all the tests pass.