refactor: usage of anyhow::context in cfg_fs block

- remove the `use anyhow::context;` from the start of the file.
- add `use anyhow::context;` within the `cfg_fs!` block.

mithril-common/src/chain_observer/model.rs

@@ -1,5 +1,4 @@
 use anyhow::anyhow;
-use anyhow::Context;
 use serde::Serialize;
 use serde_json::Value;
 use std::collections::HashMap;
@@ -10,6 +9,7 @@ use crate::{StdError, StdResult};
 
 cfg_fs! {
     use serde::Deserialize;
+    use anyhow::Context;
     use minicbor::{Decode, Decoder, decode};
     use pallas_primitives::{alonzo::PlutusData, ToCanonicalJson};
     /// [Datum] represents an inline datum from UTxO.