docs: remove unused primer ref

we already have a link to the patch primer in the main index.

Change-Id: Ib90ade76a17f5d99da8fe481d8f87c68eca38f1c
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/546
Tested-by: jenkins

doc/manual/main.txt

@@ -36,7 +36,6 @@ This pages lists Technical Primers available for OpenOCD Developers.
 They seek to provide information to pull novices up the learning curves
 associated with the fundamental technologies used by OpenOCD.
 
-- @subpage primerpatches
 - @subpage primerdocs
 - @subpage primerautotools
 - @subpage primertcl