* src/buffer.c (mode-line-format): Remove docs for obsolete %t construct

* src/buffer.c (mode-line-format): Remove docs for obsolete %t construct

This documentation had been previously removed in 1999 in [1]
but was subsequently restored again in a different form in 2006
in [2] as the construct was still supported on some systems.
However it looks like it hasn't done what that documentation
indicated since [3] in 2011, and the final remnant of this
functionality was removed in [4] in 2013; so it seems clear that
it's no longer supported and shouldn't be documented now.

[1] commit 08de62001945d4dd32a3d6af44da05804cb296d5
[2] commit dafbe726b89a36010f3f24bb1efe4ace0a5f7f0f
[3] commit 05c652517317d10690aaf0a6aa0bc876382b9d82
[4] commit 8549f9e89bd9288c4c709d183a5bf8f07dbeed3d