fn small_memset: Remove the unused UP_TO == 64 case (#1314)

It's never hit (I tried replacing the `*as_array(buf) = [val; 64]` arm
with a panic and the tests all passed). This makes sense because the
original C code only had `upto` values of 16 and 32.

src/ctx.rs

@@ -66,7 +66,6 @@ pub fn small_memset<T: Clone + Copy, const UP_TO: usize, const WITH_DEFAULT: boo
         08 if UP_TO >= 08 => *as_array(buf) = [val; 08],
         16 if UP_TO >= 16 => *as_array(buf) = [val; 16],
         32 if UP_TO >= 32 => *as_array(buf) = [val; 32],
-        64 if UP_TO >= 64 => *as_array(buf) = [val; 64],
         _ => {
             if WITH_DEFAULT {
                 buf.fill(val)