Remove unused dependency

Summary:
libcurl doesn't seem to be used anymore since
427a9ede658e4bee0e96e09741adb879e396c0c1 and the removal of the gmic
plugin.

Test Plan: Still builds fine.

Reviewers: Krita, rempt

Subscribers: woltherav

Tags: Krita

Differential Revision: https://phabricator.kde.org/D7360