GNUmakefile: Remove help

GNUmakefile: Remove help

It gets out of date easily, and it's always more accurate to actually
read the makefiles, or run a command that parses them for us.
In the case of .PHONY targets, or available variables, here are two
commands that can help:

	$ make nothing -p \
	| grep '^\.PHONY:' \
	| tr ' ' '\n' \
	| grep -v '^\.PHONY:' \
	| sort;

	$ grep -rho '^[^[:space:]].*=' GNUmakefile share/mk/configure/;

Signed-off-by: Alejandro Colomar <alx@kernel.org>