remove unnecessary Send impl on Field

zokrates_field/src/lib.rs

@@ -60,7 +60,6 @@ pub trait Field:
     + Ord
     + Display
     + Debug
-    + Send
     + Add<Self, Output = Self>
     + for<'a> Add<&'a Self, Output = Self>
     + Sub<Self, Output = Self>
@@ -152,8 +151,6 @@ mod prime_field {
                 value: BigInt,
             }
 
-            unsafe impl Send for FieldPrime {}
-
             impl Field for FieldPrime {
                 fn bits(&self) -> u32 {
                     self.value.bits() as u32