Remove inlineButton

[luixxiul]

Test plan:

#9230 (comment)

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Describe the issue you encountered: Remove inlineButton.

I agree with @cezaraugusto that inlineButton is no longer necessary. It had been applied for a cancel button on the advanced settings dialog of Brave Payments. I removed the class with PR #5805, and currently there is no button which includes inlineButton classname.

If you would like to specify display: inline to a button, let's regard the case as an exception and consider to style with inlineStyles on browserButton.js

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