gallium: Remove the pool pipebuffer manager.

gallium: Remove the pool pipebuffer manager.

Noticed while trying to decide if pipebuffer was of any use to me, and
found that nothing has used it in the last 10 years at least.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Kristian Høgsberg <hoegsberg@gmail.com>