x509-cert: don't require signature's 'derive' feature

There are no dependencies on the "derive" featue of the signature crate.
Drop it now.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

x509-cert/Cargo.toml

@@ -22,7 +22,7 @@ spki = { version = "0.7.1", features = ["alloc"] }
 # optional dependencies
 arbitrary = { version = "1.3", features = ["derive"], optional = true }
 sha1 = { version = "0.10.0", optional = true }
-signature = { version = "2.1.0", features = ["digest"], optional = true }
+signature = { version = "2.1.0", optional = true }
 
 [dev-dependencies]
 hex-literal = "0.4"