Remove unneeded vector element Sized constraints

We only implement VectorElement for sized types anyway.
diff --git a/src/cxx_vector.rs b/src/cxx_vector.rs
index c3f4fe7..c0d2f6b 100644
--- a/src/cxx_vector.rs
+++ b/src/cxx_vector.rs
@@ -68,15 +68,9 @@
 // codebase.
 #[doc(hidden)]
 pub unsafe trait VectorElement<T> {
-    fn __get_unchecked(v: &CxxVector<T>, pos: usize) -> &T
-    where
-        Self: Sized;
-    fn __vector_length(v: &CxxVector<T>) -> usize
-    where
-        Self: Sized;
-    fn __push_back(v: &CxxVector<T>, item: &T)
-    where
-        Self: Sized;
+    fn __get_unchecked(v: &CxxVector<T>, pos: usize) -> &T;
+    fn __vector_length(v: &CxxVector<T>) -> usize;
+    fn __push_back(v: &CxxVector<T>, item: &T);
 }
 
 cxxbridge_macro::vector_builtin!(u8);