Remove incorrect function import from pgindent

Commit 149ac7d455 which re-implemented pgindent in Perl explicitly
imported the devnull function from File::Spec, but the module does
not export anything.  In recent versions of Perl calling a missing
import function cause a warning, which combined with warnings being
fatal cause pgindent to error out.

Backpatch to all supported versions.

Author: Erik Wienhold <ewie@ewie.name>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discusson: https://postgr.es/m/2372cd74-11b0-46f9-b28e-8f9627215d19@ewie.name
Backpatch-through: v12

src/tools/pgindent/pgindent

@@ -7,7 +7,7 @@ use warnings;
 
 use Cwd qw(abs_path getcwd);
 use File::Find;
-use File::Spec qw(devnull);
+use File::Spec;
 use File::Temp;
 use IO::Handle;
 use Getopt::Long;