Remove fixme of packet "k" from doc
https://sourceware.org/ml/gdb-patches/2014-03/msg00324.html

2014-03-21  Pedro Alves  <palves@redhat.com>
	    Stan Shebs  <stan@codesourcery.com>
	    Hui Zhu  <hui@codesourcery.com>

	* gdb.texinfo (Packets): Add anchor to "? packet".
	Remove fixme and update introduction of "k packet".
	Add anchor to "vKill packet".