docs($http): remove outdated part about $http outside of $apply phase

This removes some outdated advice which no longer is true against the latest angular version.

The information about unit testing with ngMocks remains, because it's always good to have
information like that easily found. This little snippet is not worded perfectly, and is not
a very good example unit test, so additional work is needed here.

Relates to #5206
Closes #5485