dirstate: move most of the `remove` logic with dirstatemap `removefile` This code deal with special logic to preserving "merged" and "from_p2" information when removing a file. These are implementation details that are more suitable for the dirstatemap layer. Since the dirstatemap layer alreaday have most of the information necessary to do so, the move is easy. This move helps us to encapsulate more implementation details within the dirstatemap and its entry. Easing the use of a different storage for dirstate v2. Differential Revision: https://phab.mercurial-scm.org/D10953