build: Remove obsolete iree flags

Remove two obsolete iree flags IREE_TARGET_BACKENDS_TO_BUILD and
IREE_HAL_DRIVERS_TO_BUILD. They were removed from IREE codebase at
https://github.com/openxla/iree/pull/7936/.

Change-Id: If505172caca03a0c5ee4c922ad4cc8548760a013
2 files changed