`metadata add --clear-authors` doesn't clear authors

```
$ cat foo1.wat
(component $foo
  (core module (;0;)
    (type (;0;) (func))
    (func $foo (;0;) (type 0))
  )
  (@custom "authors" "J. Random Hacker")
)
$ wasm-tools metadata add --clear-authors foo1.wat -t
(component $foo
  (core module (;0;)
    (type (;0;) (func))
    (func $foo (;0;) (type 0))
  )
  (@custom "authors" "J. Random Hacker")
)
```