Remove deprecated Repository.mapTree

Remove deprecated Repository.mapTree

The mapTree() routines have been deprecated for a long time, and their
sibilings for mapCommit() and mapTag() were already removed from the
main Repository API.

Remove mapTree().  Application callers who only need the tree's name
can use resolve("^{tree}") syntax to resolve to the tree ObjectId, or
fail if the input is not a tree.

Applications that want to read a tree should use DirCache or TreeWalk.

Change-Id: I85726413790fc87721271c482f6636f81baf8b82
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>