Fix a warning in Perl test code

The code was passing a scalar argument to node->restart(), but it was
expecting a hash, which causes a warning from Perl ("Odd number of
elements in hash assignment").

But the node->restart() function doesn't take a mode argument anyway.
This was probably copied from an incorrect comment (see commit
750c59d7ec).  The default restart mode is already "fast", so the test
should still be semantically correct without explicitly specifying the
mode.

Discussion: https://www.postgresql.org/message-id/e3f4bf1b-63d3-408a-b07e-d35a0fdf1b98@eisentraut.org

src/test/recovery/t/006_logical_decoding.pl

@@ -70,7 +70,7 @@ is(scalar(my @foobar = split /^/m, $result),
 # If we immediately crash the server we might lose the progress we just made
 # and replay the same changes again. But a clean shutdown should never repeat
 # the same changes when we use the SQL decoding interface.
-$node_primary->restart('fast');
+$node_primary->restart;
 
 # There are no new writes, so the result should be empty.
 $result = $node_primary->safe_psql('postgres',