transfer: remove 'uploadbuf' pointer and cleanup readwrite_upload()

The data->req.uploadbuf struct member served no good purpose, instead we
use ->state.uploadbuffer directly. It makes it clearer in the code which
buffer that's being used.

Removed the 'SingleRequest *' argument from the readwrite_upload() proto
as it can be derived from the Curl_easy struct. Also made the code in
the readwrite_upload() function use the 'k->' shortcut to all references
to struct fields in 'data->req', which previously was made with a mix of
both.