remove the Mutex.borrow_mut

because it can be used to bypass Rust's borrow checker

``` rust
static FOO: Mutex<bool> = Mutex::new(false);

fn main() {
    cortex_m::interrupt::free(|mut cs1| {
        cortex_m::interrupt::free(|mut cs2| {
            let foo = FOO.borrow_mut(&mut cs1);
            let and_foo = FOO.borrow_mut(&mut cs2);
        });
    });
}
```

1 files changed, 0 insertions, 8 deletions

diff --git a/src/interrupt.rs b/src/interrupt.rs
index e11fdc3..a96b845 100644
--- a/src/interrupt.rs
+++ b/src/interrupt.rs
@@ -19,14 +19,6 @@ impl<T> Mutex<T> {
     pub fn borrow<'cs>(&self, _ctxt: &'cs CriticalSection) -> &'cs T {
         unsafe { &*self.inner.get() }
     }
-
-    /// Mutably borrows the data for the duration of the critical section
-    pub fn borrow_mut<'cs>(
-        &self,
-        _ctxt: &'cs mut CriticalSection,
-    ) -> &'cs mut T {
-        unsafe { &mut *self.inner.get() }
-    }
 }
 
 /// Interrupt number