Remove XCF support

Dmitry Kazakovrequested to merge
dkazakov/krita:kazakov/remove-xcf-support into master

The library we used for opening XCF files (xcftools [1]) is currently abandoned. It has several open CVE reports open. We tried to keep them fixed by forking the library locally and adding a fix for them. But now we got three CVE reports against this forked library (and against our seemingly incomplete fixes). And we have no resources to keep this fork safe.

[1] - https://github.com/j-jorge/xcftools

Edited by Dmitry Kazakov

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