Remove unnecessary `startedTextExtraction` member variable from the find controller
Remove unnecessary `startedTextExtraction` member variable from the find controller

The find controller already has quite a lot of state to maintain. We can
avoid keeping track of this member variable because when the find
controller is reset, so is the extract text promises array. Therefore,
we can just check if that array contains items or not to determine if
text extraction already started.

Moreover, there is no need to reset the `pageContents` array since the
`reset` method already takes care of that.

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