Removes the Browse Extensions button in the default configuration.

This is a fix for #6800. If the extension_wiki_url configuration option is not
set, the Browse Extensions button will not be displayed. In the default Brackets
configuration, this value is no longer set.
Addresses review comments for browse extensions button.
Merge pull request #6908 from adobe/dangoor/6800-browse-extensions

Removes the Browse Extensions button in the default configuration.