zsh: fix invalid postrm script and little refactor of scripts

zsh: fix invalid postrm script and little refactor of scripts

The postrm script was missing shebang. Postrm scripts are packaged and
executed directly and not sourced by default script (as in case of prerm
and postinst).

Also move some indents around to not confuse reader. The section in
postinst was indented to same level as grep "condition" but is on same
level as initial grep (not part of that "condition").

Signed-off-by: Karel Kočí <redacted>