Limit content of web browser version of http-strings to web browser.

In general it seems a bad idea to have two http-strings.c files as this precludes to have them both in the Contiki library. However as it stands it seems most reasonable to have one http-strings.c file be a clean superset of all usecases in order to allow them to run together in a single binary. As webserver/http-strings.c already contained strings not present in webbrowser/http-strings.c it seems reasonable to consider webserver/http-strings.c as the superset described. From that perspective it is appropriate to remove all strings from webbrowser/http-strings.c which are not used by the web browser in order to save memory otherwise wasted.