I doubt that you would be in any trouble for offering an ad-free site for users who subscribe or log in. This is fairly common. Deciding to not show ads at all is at your discretion.
Google's AdSense program policies are focused toward preventing invalid and incentivized clicks. Your scenario is not mentioned at all.
The relevant parts:
Invalid clicks and impressions
Publishers may not click their own ads or use any means to inflate impressions and/or clicks artificially, including manual methods.
Clicks on Google ads must result from genuine user interest. Any method that artificially generates clicks or impressions on your Google ads is strictly prohibited. These prohibited methods include, but are not limited to, repeated manual clicks or impressions, automated click and impression generating tools and the use of robots or deceptive software. Please note that clicking your own ads for any reason is prohibited.
Publishers may not ask others to click their ads or use deceptive implementation methods to obtain clicks. This includes, but is not limited to, offering compensation to users for viewing ads or performing searches, promising to raise money for third parties for such behavior or placing images next to individual ads.
In order to ensure a good experience for users and advertisers, publishers participating in the AdSense program may not:
- Compensate users for viewing ads or performing searches, or promise compensation to a third party for such behavior.
- Encourage users to click the Google ads using phrases such as "click the ads", "support us", "visit these links" or other similar language.
- Direct user attention to the ads using arrows or other graphical gimmicks.
- Place misleading images alongside individual ads.
- Place ads in a floating box script.
- Format ads so that they become indistinguishable from other content on that page.
- Format site content so that it is difficult to distinguish it from ads.
- Place misleading labels above Google ad units. For instance, ads may be labelled "Sponsored Links" or "Advertisements", but not "Favourite Sites" or "Today's Top Offers".
In fact, Google now has a program called Contributor where people can pay a monthly fee and be shown fewer ads. The amount of money they pay is used in the live ad auction to bid against other ads, and when their automatic bid wins for an ad unit, contributors see a "Thanks for contributing" block instead of an ad. You receive the same percentage of that monthly fee as you would for the ads that would otherwise have been shown.