rust-agent: Remove unused functions

Fixes the following warning:

   Compiling logging v0.1.0 (/home/ddd/go/src/github.com/kata-containers-2.0/pkg/logging)
   warning: associated function is never used: `set_level`
      --> /home/ddd/go/src/github.com/kata-containers-2.0/pkg/logging/src/lib.rs:186:8
       |
   186 |     fn set_level(&self, level: slog::Level) {
       |        ^^^^^^^^^
       |
       = note: `#[warn(dead_code)]` on by default

Fixes: #750

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

pkg/logging/src/lib.rs

@@ -182,12 +182,6 @@ impl<D> RuntimeLevelFilter<D> {
             level: Mutex::new(level),
         }
     }
-
-    fn set_level(&self, level: slog::Level) {
-        let mut log_level = self.level.lock().unwrap();
-
-        *log_level = level;
-    }
 }
 
 impl<D> Drain for RuntimeLevelFilter<D>