tests: net: http_server: Remove harness requirement

The HTTP server tests are self-contained, they do not require network
environment to execute, hence should not specify "harness: net".

The consequence of specifying the harness was that HTTP server tests in
the CI were only built, and not executed, which doesn't make much sense.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>