remove endless httr imports

rsunlight is dependent on httr. A vast number of functions contain import tags in their documentation that import httr, as a result. This is, however, inconsistent (some functions that use it, don't document that they use it), unnecessary (all querying has been moved to query() now anyway) and duplicative (one import tag is quite enough to have roxygen2 modify the NAMESPACE file appropriately).

This change removes all of the <code>@import httr</code> lines from rsunlight, replacing them with a single import tag in the rsunlight-package documentation.