Bug#1084857: magic-wormhole: drop spurious hkdf dependency

[Chris Hofstaedtler]

Source: magic-wormhole
Version: 0.14.0-0.1
Severity: important
Control: block 1082104 by -1

Upstream switched from hkdf to cryptography in 0.13.0. Please drop
the python3-hkdf dep, as it was requested to be removed.

Chris