Docs: remove instructions which are already default in task-color.5

The text even mentions, that it might not be necessary. So just remove
this section.

doc/man/task-color.5.in

@@ -3,20 +3,6 @@
.SH NAME
task-color \- A color tutorial for the Taskwarrior command line todo manager.

.SH SETUP
The first thing you need is make sure you tell Taskwarrior that you want to use
color.  This is the default for Taskwarrior, so the following step may be
unnecessary.

    $ task config color on

This command will make sure there is an entry in your ~/.taskrc file that looks
like:

    color=on

Now Taskwarrior is ready.

.SH AUTOMATIC MONOCHROME
It should be mentioned that Taskwarrior is aware of whether its output is going
to a terminal, or to a file or through a pipe.  When Taskwarrior output goes to