bug#18647: 24.4.50; tramp: keep your hands off vc-handled-backends!

[Sam Steingold]

Every now and then I get the error "file is not under version control"
because tramp removes stuff from `vc-handled-backends' when it cannot
detect the appropriate executable on the remote host.

There are two problems here: first, minor - this is global: if tramp
cannot find git on host X, I will not be able to use git on _any_ host
including localhost.

Second, critical - my laptop has internet connection only
intermittently, so whenever I lose connection, tramp cannot find git on
a remote host, and I have to re-add stuff to `vc-handled-backends'.

Proposal: remove the offending code in
tramp-sh.el:tramp-sh-handle-vc-registered.

Thanks!

In GNU Emacs 24.4.50.6 (x86_64-apple-darwin13.3.0, NS appkit-1265.21 Version 
10.9.5 (Build 13F34))
 of 2014-09-22 on sds-MacBook-Pro.local
Windowing system distributor `Apple', version 10.3.1265
Configured using:
 `configure --with-ns'

Configured features:
IMAGEMAGICK ACL GNUTLS LIBXML2 ZLIB

Important settings:
  value of $LANG: C
  locale-coding-system: utf-8-unix

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1265
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