OvmfPkg: remove BLOCK_MMIO_PROTOCOL and BlockMmioToBlockIoDxe

OvmfPkg: remove BLOCK_MMIO_PROTOCOL and BlockMmioToBlockIoDxe

BLOCK_MMIO_PROTOCOL and BlockMmioToBlockIoDxe were introduced to OvmfPkg
in March 2010, in adjacent commits b0f5144676fa and efd82c5794ec. In the
past eight years, no driver or application seems to have materialized that
produced BLOCK_MMIO_PROTOCOL instances. Meanwhile the UEFI spec has
developed the EFI_RAM_DISK_PROTOCOL, which edk2 implements (and OVMF
includes) as RamDiskDxe.

Rather than fixing issues in the unused BlockMmioToBlockIoDxe driver,
remove the driver, together with the BLOCK_MMIO_PROTOCOL definition that
now becomes unused too.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Steven Shi <steven.shi@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=926
Reported-by: Steven Shi <steven.shi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>