remove usk_to_tkey #120

Job	Run time
Test on Linux	1s
Test Orchard on Linux	1s
Bitrot check	3m 5s
Latest build on ubuntu-latest	2m 28s
Test NU6 on Linux	1s
Build target wasm32-wasi	2m 50s
Latest build on windows-latest	7m 5s
Test on macOS	5m 4s
Latest build on macOS-latest	1m 41s
Test Orchard on macOS	1s
Check ZFuture build on Linux	4m 5s
Test on Windows	1s
Check ZFuture build on macOS	3m 7s
Test Orchard on Windows	1s
Check ZFuture build on Windows	7m 4s
Clippy (MSRV)	3m 5s
Test NU6 on Windows	1s
Clippy (beta)	14s
Code coverage	10m 22s
Intra-doc links	3m 37s
Rustfmt	18s
protobuf consistency	2m 59s
UUID validity	7s
	57m 18s

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