remove assertion

Apparently, we actually hit this when the user explicitly enables vectored
writes but  the IO doesn't return `true` from `is_write_vectored`.

This kind of begs the question, do those APIs still make sense? Is there
ever a reason to enable writev on an IO that doesn't return `true` from
`is_write_vectored`? I think that *disabling* vectored writes explicitly
when the IO supports them makes sense, but I'm unconvinced about
*enabling* them when it doesn't.

@seanmonstar any thoughts?

Signed-off-by: Eliza Weisman <[email protected]>

Author: hawkw

src/proto/h1/io.rs

@@ -241,11 +241,6 @@ where
             }
 
             loop {
-                debug_assert!(
-                    self.io.is_write_vectored(),
-                    "using vectored writes on an IO that does not provide fast vectored \
-                     write support, this is a bug"
-                );
                 let n = {
                     let mut iovs = [IoSlice::new(&[]); MAX_WRITEV_VECS];
                     let len = self.write_buf.bytes_vectored(&mut iovs);