SoupHTTPInputStream: remove GSeekable support
SoupHTTPInputStream: remove GSeekable support

This was needed in gvfs (where this code originally came from), but
not here.

Distrotech/libsoup,Distrotech/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,Distrotech/libsoup,Distrotech/libsoup,jwendell/libsoup,Distrotech/libsoup,fanc999/libsoup,fanc999/libsoup