icon format adjustment no longer needed

In sed commands (multiple instances), -e 's|\.png|\.xpm|' to adjust icon in desktop file appears to no longer be needed for current versions of Lazarus. The desktop file does not reference png. Even if it did, the icon is in png format.

As-is, that portion of the sed command doesn't do anything, so could be removed with a future release.

For reference, the upstream lazarus.desktop for version 4.0.