Replace with `strings.TrimPrefix`

Instead of using `strings.HasPrefix` and manual slicing, use the
`strings.TrimPrefix` function. If the string doesn't start with the
prefix, the original string will be returned. Using
`strings.TrimPrefix` reduces complexity, and avoids common bugs, such
as off-by-one mistakes.

**Before:**

```
if strings.HasPrefix(str, prefix) {
  str = str[len(prefix):]
}
```

**After:**

```
str = strings.TrimPrefix(str, prefix)
```
