internal/extsvc/github: Check status code only for conditional request
Created by: bobheadxi
We no longer set the headerIfNoneMatch for conditional requests since we let the httpcache take care of etags and adding the appropriate header1. As a result we should not check for this either since the check fails for conditional requests and ends up in an EOF error with the body being empty.
Test plan
This commit is extracted from https://github.com/sourcegraph/sourcegraph/pull/34004 , where there are a lot of testing efforts going on. I'm pretty confident the behaviour is correct, namely with https://github.com/sourcegraph/sourcegraph/pull/34004#discussion_r853523786 https://github.com/sourcegraph/sourcegraph/commit/b3a0e430cc0526a975038c827769e9bd48a0971b#diff-ee6bd73704b29750eb3d94d6068033609be3cb2d5052506f176aa5adb5ab09d9L82