Fix removing element in 'trusted-create-element'

[AdamWr]

It looks like that if cleanupDelayMs parameter is used, then removed element is added and removed again few times.

Steps to reproduce:

1. Add this rule:

example.org#%#//scriptlet('trusted-create-element', 'body', 'div', '', '', '1000')

2. Go to - https://example.org/

Added div should be removed after 1 second, but it's added again few times and then removed. I guess that we should disconnect MutationObserver after removing element.