Disable slasher default features (#6115)

* Turn off default features for slasher.

Because slasher had lmdb as a default feature, it was impossible to have
a pure mdbx build of lighthouse. This commit sets
`default-features = false` for slasher so lighthouse can actually choose
one or the other.

Cargo.toml

@@ -224,7 +224,7 @@ pretty_reqwest_error = { path = "common/pretty_reqwest_error" }
 proto_array = { path = "consensus/proto_array" }
 safe_arith = { path = "consensus/safe_arith" }
 sensitive_url = { path = "common/sensitive_url" }
-slasher = { path = "slasher" }
+slasher = { path = "slasher", default-features = false }
 slashing_protection = { path = "validator_client/slashing_protection" }
 slot_clock = { path = "common/slot_clock" }
 state_processing = { path = "consensus/state_processing" }