dim: Drop support for git < 2.5 (without worktree)

git 2.5 is too old to say we really support it. Supporting git older
than that is a burden and we are likely to break it without noting.
We even already check for git >= 2.8. Just stop checking for `git
worktree` availability.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>