Revert "Consolidate bash version checks"

This reverts commit 41a6899538a8f91629897fc61a5740238e3927b6.
Revert "Check bash version instead of OS for bash completion"

This reverts commit 59befe88c36dc14f7fc2ad4e653f50203bb225bd.
Revert "The bash completion stuff is crapping out on darwin. Disabling."

This reverts commit a5eeb6a88fc6122d38534bc8ce71038d082ad198.
Revert "envsetup: include bash completion script from vendor/crom"

This reverts commit 8a37be62d2c2007e2494f04b62dc1d886180e293.