add: use advise_if_enabled for ADVICE_ADD_IGNORED_FILE

add: use advise_if_enabled for ADVICE_ADD_IGNORED_FILE

Since b3b18d1621 (advice: revamp advise API, 2020-03-02), we can use
advise_if_enabled() to display an advice.  This API encapsulates three
actions:
	1.- checking the visibility of the advice

	2.- displaying the advice when appropriate

	3.- displaying instructions on how to disable the advice, when
	    appropriate

The code we have in builtin/add.c to display the ADVICE_ADD_IGNORED_FILE
advice, is doing these three things.  However, the instructions
displayed on how to disable the hint are not shown in the normalized way
that advise_if_enabled() introduced.  This may cause distraction.

There is no reason not to use the new API here.  On the contrary, by
using it we gain simplicity in the code and avoid possible distractions.

For these reasons, use the newer advise_if_enabled() machinery to show
the ADVICE_ADD_IGNORED_FILE advice, and don't bother checking the
visibility or displaying the instruction on how to disable the advice.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>