Revert the mirroring of SoupMessage API onto SoupRequestHTTP.

Revert the mirroring of SoupMessage API onto SoupRequestHTTP.

SoupMessage isn't being deprecated, and mirroring its API onto
SoupRequestHTTP is just going to result in always having to add every
new API twice. Also, it turns out to be less useful than originally
expected anyway, since you end up having to cast between SoupRequest
and SoupRequestHTTP frequently anyway.

This reverts commit d7117329400e47d2187ed033099d921d555f8d71 and most of
commit 53c270d0e2868fa5ad48ce864f10a9486b11a071.