Fix stray rm (of an undefined variable).

diff -Naur arpwatch-3.1-original/arpfetch arpwatch-3.1/arpfetch
--- arpwatch-3.1-original/arpfetch	2013-02-16 03:10:28.000000000 -0500
+++ arpwatch-3.1/arpfetch	2020-11-07 11:22:59.344575624 -0500
@@ -29,5 +29,3 @@
 	    ea = $2
 	    print ea "\t" ip
     }'
-
-rm -f ${t1}