internal/frontend: remove unused field: addDocQueryParam

Remove unused field: addDocQueryParam, which was caught
by this staticcheck error:
internal/frontend/server_test.go:72:2: field addDocQueryParam is unused (U1000)

Change-Id: I13680024e7ec0011a0af1feb7424fdf83ea72fce
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/241319
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: julieqiu

internal/frontend/server_test.go

@@ -68,8 +68,6 @@ type serverTestCase struct {
 	name string
 	// path to use in an HTTP GET request
 	urlPath string
-	// whether to mutate the identifier links in documentation.
-	addDocQueryParam bool
 	// statusCode we expect to see in the headers.
 	wantStatusCode int
 	// if non-empty, contents of Location header. For testing redirects.