doc/manual/primer/autotools.txt: fix doxygen warning

doc/manual/primer/autotools.txt: fix doxygen warning

Commit ab90b8777855 ("configure: remove AM_MAINTAINER_MODE,
effectively always enabling all the rules") removes the configure
flag '--enable-maintainer-mode' and its documentation, but have
left a reference to the removed subsection 'primermaintainermode'
and this triggers a warning in doxygen:
	doc/manual/primer/autotools.txt:21: warning: unable to
	resolve reference to 'primermaintainermode' for \ref
	command

Remove the obsoleted paragraph.

Change-Id: I56e69ef033d546d159745bed1b47c6105827e7ae
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: ab90b8777855 ("configure: remove AM_MAINTAINER_MODE, effectively always enabling all the rules")
Reviewed-on: http://openocd.zylin.com/6003
Tested-by: jenkins