external-sourcery-toolchain: remove

external-sourcery-toolchain: remove

As per discussion on the mailing list [1], remove this largely
unmaintained external toolchain support in favour of the maintained
version in meta-sourcery [2].

Also correct the example and documentation.conf entries for TCMODE to
match up with this change.

[1] http://lists.openembedded.org/pipermail/openembedded-core/2013-December/087133.html
[2] https://github.com/MentorEmbedded/meta-sourcery/

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>