Since as far as I can see nobody has explained how to delete apps from the watch, here's the easiest way to do it:
In the Watch app on your phone, scroll down to the list of apps under "Installed on Apple Watch." Tap on the right arrow for the app and set the green slider to the left. It will then be uninstalled on the watch but remain on your phone, and added to the list of "Available Apps" at the bottom of the list.
Personally I would advise against removing all third party apps. Even if this corrects your battery draining issue, it means you will still not know which app was misbehaving. Taking away one or two at a time and seeing if it helps gives you a much better chance of finding the problem. Generally trying one thing at a time is a preferable diagnostic approach. The blunderbuss approach may fix the problem, but you'll never know why.
BTW, installing watchOS 10.1 seems to have corrected the battery drain issue on my Series 5. So probably a lot of us can simply do this and forego the other steps.