Remove incorrect function import from pgindent

Remove incorrect function import from pgindent

Commit 149ac7d4559 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