d/postfix.postinst: Use sed instead of perl

postfix.postinst use Perl to remove stray mynetworks configuration lines. The specific use of perl in this maintscript can be easily replaced with an equivalent sed invocation.

Using sed instead of Perl reduces the number of packages required to install postfix. (Perl is a candidate for removal from essential, see https://salsa.debian.org/gioele/debian-vm-minimal-pkgs and https://wiki.debian.org/Proposals/EssentialOnDiet.)