printk-formats.txt: remove unimplemented %pT

%pT for task->comm has been proposed (several times, I think), but is
not actually implemented.  Remove it from printk-formats.txt and add it
back if/when it gets implemented.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>