Installer: get rid of workaround for udisks bug#418 in SetFlags()

We’ve been workaround’ing https://github.com/storaged-project/udisks/issues/418 for years that just got fixed in udisks upstream… less than 2 months after I’ve reported it. I don’t think it’s worth pushing the fixes (that lives in libblockdev) to Stretch & other LTS distros we support, but once all distros we want to support have udisks2 2.7.5+ we can finally get rid of this workaround.

Related issues

Blocked by #15944 (closed)

Original created by @intrigeri on 15010 (Redmine)

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