Clean up ls command

Problem

With duo-workflow-service!338 (merged) we now have 2 branches for listing files.

1. git ls-tree
2. ls

This is controlled with FEATURE_GIT_LS_TREE_INSTEAD_OF_LS env variable.

If git ls-tree is stable, we should clean-up ls.

Desired Outcome

1. ls command is removed.
2. FEATURE_GIT_LS_TREE_INSTEAD_OF_LS is removed

Implementation Plan