zcash_note_encryption: Clean up dependencies

Several dependencies were copied over during the extraction of this
crate's logic from `zcash_primitives`, but are in fact only required for
the protocol-specific logic. We can also remove the `std` feature flag,
since we no longer have a dependency on `blake2b_simd` that needs its
`std` flag exposed for performance.

components/zcash_note_encryption/Cargo.toml

@@ -16,24 +16,20 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
-blake2b_simd = { version = "0.5", default-features = false }
-byteorder = { version = "1", default-features = false }
 chacha20 = { version = "0.8", default-features = false }
 chacha20poly1305 = { version = "0.9", default-features = false }
-ff = { version = "0.11", default-features = false }
-group = { version = "0.11", default-features = false }
 rand_core = { version = "0.6", default-features = false }
 subtle = { version = "2.2.3", default-features = false }
 
 [dev-dependencies]
+ff = { version = "0.11", default-features = false }
 zcash_primitives = { version = "0.5", path = "../../zcash_primitives" }
 jubjub = "0.8"
 
 [features]
-default = ["std"]
+default = ["alloc"]
 alloc = []
 pre-zip-212 = []
-std = ["alloc", "blake2b_simd/std"]
 
 [lib]
 bench = false