Only delete A RR, not any RR for the FQDN

Make the delete command specific to A RRs.  This prevents ddclient from
deleting other RRs unrelated to the dynamic address, but on the same
FQDN.  This can be specifically a problem with KEY RRs when using SIG(0)
instead of symmetric keys.

Reported by:	Wellie Chao
Bug report:	http://sourceforge.net/p/ddclient/bugs/71/