unpack_strategy/dmg: remove deprecated IDME attach flag

[miccal]

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On Catalina the IDME attach flag has been removed -- according to man hdiutil:

Removed the deprecated "hdiutil internet-enable" command and the IDME attach flags.

and when installing a Cask that used a .dmg the following message is printed:

hdiutil: attach: WARNING: ignoring IDME options (obsolete)

Thank you.

[miccal]

Thank you @reitermarkus.