luci-app-package-manager: fix 'autoremove' checkbox uneditable

luci-app-package-manager: fix 'autoremove' checkbox uneditable

In HTML, the following lines of code are equivalent:
```
<input disabled />
<input disabled="true" />
<input disabled="false" />
<input disabled="any" />
```
If you want to cancel the 'disabled' flag, you must completely remove it from the HTML instead of assigning a value.

Signed-off-by: Liangbin Lian <[email protected]>