Revert "default krb_rdns to True"

This reverts commit 02128ee577.

The second column in cfgmap is an alias and "True" is not a valid alias.
It was mistaken in the reverted commit as a default value, which it is
not. Reverting this commit should fix the following traceback that
koji-gc in Fedora currenty raises:

Traceback (most recent call last):
File "/usr/sbin/koji-gc", line 901, in <module>
options, args = get_options()
File "/usr/sbin/koji-gc", line 159, in get_options
if config.has_option(*alias):
TypeError: has_option() argument after * must be a sequence, not bool

util/koji-gc

@@ -131,7 +131,7 @@ def get_options():
             ['keytab', None, 'string'],
             ['principal', None, 'string'],
             ['krbservice', None, 'string'],
-            ['krb_rdns', True, 'boolean'],
+            ['krb_rdns', None, 'boolean'],
             ['runas', None, 'string'],
             ['user', None, 'string'],
             ['password', None, 'string'],