On Ubuntu 22.04, Browse could not load any web page except the local
start page.  Also, the WebKitGTK MninBrowser was also affected, if run
within Sugar Terminal.

Delete the proxy environment variables rather than set them empty.

Signed-off-by: James Cameron <quozl@laptop.org>

Closes #962
Closes https://github.com/sugarlabs/browse-activity/issues/119