docs: remove description of de-ANSI-fication support from manual

docs: remove description of de-ANSI-fication support from manual

* doc/automake.texi (Auxiliary Programs): Remove mention of
`ansi2knr.c' and `ansi2knr.1'.
(Obsolete Macros): Remove mention of `AM_C_PROTOTYPES'.  Do not
index it nor variables `ANSI2KNR' and `U' anymore.
(List of Automake options): Remove mention of `ansi2knr' option,
and obsolete entries for concept and option indexes.
(Optional): Remove entry about `AM_C_PROTOTYPES'.
(@c LocalWords): Remove `ansi' and `knr'.
(@menu, @detailmenu): Remove entries about de-ANSI-fication
support.