* 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 08de620019
[2] commit dafbe726b8
[3] commit 05c6525173
[4] commit 8549f9e89b

src/buffer.c

@@ -5143,7 +5143,6 @@ A string is printed verbatim in the mode line except for %-constructs:
   %P -- print percent of buffer above bottom of window, perhaps plus Top,
         or print Bottom or All.
   %n -- print Narrow if appropriate.
-  %t -- visited file is text or binary (if OS supports this distinction).
   %z -- print mnemonics of keyboard, terminal, and buffer coding systems.
   %Z -- like %z, but including the end-of-line format.
   %e -- print error message about full memory.