Simplify postprocess_pot_file

Summary:
The behavior of sed -i is similar to what we were doing here manually.

It assumes we are using GNU sed (https://en.wikipedia.org/wiki/Sed#In-place_editing), is that OK to assume?

The commented out code has been commented out since 2006, so it seems safe to remove.

Test Plan: Untested, assumed to work.

Reviewers: aacid, ltoscano

Reviewed By: aacid

Differential Revision: https://phabricator.kde.org/D24411