Use generic vc functions for both SVN and Git

eide-vc.el was designed to support a project registered in both SVN and
Git. In practice, it never happens. SVN and Git are usually mixed with
"git svn" commands, which results in a Git only repository.

The code can be simplified if we don't need to check all VC backends
anymore.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Reduce time spent to check VC status

When checking the VC status of a file, it is not necessary:
- To check if the file exists: vc-state will not fail if it does not
  exist.
- To check if the file is registered in a version control system:
  vc-state already checks that.

Removing these tests improves startup time when many files are opened.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>