insights: Consider removing not fully implemented webhook support

Created by: slimsag

Since we're going to do a push model https://github.com/sourcegraph/sourcegraph/issues/17220 there is some preliminary webhook support stuff we can remove (not a ton of code, removing may make sense to simplify):

https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+lang:go+file:insights+webhook&patternType=literal