fix: remove #[used] from non-static fields

Rust 1.51 has a regression restricting the `#[used]` attribute[0] to
static fields. As such, it's not possible to build `weechat-matrix-rs`
on stable or nightly Rust. According to the `#[used]` RFC[1], the
attribute is for static variables so it's unclear if the new behavior is
the correct one.

However, to prevent clippy from complaining during CI, replace with
`#[allow(dead_code)]`

[0] rust-lang/rust#82668
[1] https://rust-lang.github.io/rfcs/2386-used.html

src/bar_items/mod.rs

@@ -10,11 +10,11 @@ use buffer_plugin::BufferPlugin;
 use status::Status;
 
 pub struct BarItems {
-    #[used]
+    #[allow(dead_code)]
     status: BarItem,
-    #[used]
+    #[allow(dead_code)]
     buffer_name: BarItem,
-    #[used]
+    #[allow(dead_code)]
     buffer_plugin: BarItem,
 }
 

src/completions.rs

@@ -11,7 +11,7 @@ use weechat::{
 use crate::Servers;
 
 pub struct Completions {
-    #[used]
+    #[allow(dead_code)]
     servers: CompletionHook,
 }
 

src/lib.rs

@@ -150,15 +150,14 @@ impl SignalCallback for Servers {
 
 struct Matrix {
     servers: Servers,
-    #[used]
+    #[allow(dead_code)]
     commands: Commands,
-    #[used]
     config: ConfigHandle,
-    #[used]
+    #[allow(dead_code)]
     bar_items: BarItems,
-    #[used]
+    #[allow(dead_code)]
     typing_notice_signal: SignalHook,
-    #[used]
+    #[allow(dead_code)]
     completions: Completions,
     debug_buffer: RefCell<Option<BufferHandle>>,
 }