If I remove an app from an app group, will it delete it from the devices?
No. App Groups are responsible for distributing apps to devices (device groups, to be specific). Removing an app from an app group and pushing the apps will not cause already-distributed apps to delete from devices.
If you want to delete an app from a device, you can do so by going to the 'Apps' tab of the Device Details page for a given device. A listing of all the installed apps on a device will be displayed. Apps that were installed with SimpleMDM can optionally be removed. This option is not available for macOS.
Alternatively, to remove a single app from multiple devices: go to Apps > Catalog, click the app name, click the 'Installs' tab, check the box next to the device name(s), click 'Actions', and select 'Uninstall'.