Remove comments about vc-bzr.el being a modified unofficial version.

Remove comments about vc-bzr.el being a modified unofficial version.
(vc-bzr-command): Remove redundant setting of process-connection-type.
(vc-bzr-admin-checkout-format-file): Add autoload.
(vc-bzr-root-dir): Remove in favor of vc-bzr-root.
(vc-bzr-root): Switch to implementation of vc-bzr-root-dir.
(vc-bzr-registered): Compare dirstate format tag with known good
value, abort parsing if match fails.  Warn user in docstring.
(vc-bzr-workfile-version): Case for different Bzr branch formats.
See bzrlib/branch.py in Bzr sources.
(vc-bzr-diff): First argument FILES may be a string rather than a list.
(vc-bzr-shell-command): Remove in favor of
vc-bzr-command-discarding-stderr.
(vc-bzr-command-discarding-stderr): New function.