migration: Revert postcopy-blocktime commit set

This reverts commits
ca6011c2 migration: add postcopy total blocktime into query-migrate
5f32dc8e migration: add blocktime calculation into migration-test
2f7dae9d migration: postcopy_blocktime documentation
3be98be4 migration: calculate vCPU blocktime on dst side
01a87f0b migration: add postcopy blocktime ctx into MigrationIncomingState
31bf06a9 migration: introduce postcopy-blocktime capability

as they don't build on ppc32 due to trying to do atomic accesses
on types that are larger than the host pointer type.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>