Partial revert of
r37016 (attempt to swap out old compat wfGetHTTP() with Http::get())
function_exists( 'Http::get' ) always returns false, since that's not a function name.
If the current state of the extension only works on modern versions of MW, then just call Http::get() without checking. If it still works on versions that predate it, it's best to keep the existing compat code probably.