Revert all bash completion stuff

Revert "Consolidate bash version checks"

This reverts commit 4e0a47cbd3198ee4ba38839654c79e7c8726cf90.

Revert "Check bash version instead of OS for bash completion"

This reverts commit 2812e6e6e93f6d71221f6da676b84960ed68e7ea.

Revert "The bash completion stuff is crapping out on darwin. Disabling."

This reverts commit bc6d7c36be0883af10784c854eb808a0d313c39c.

Revert "envsetup: include bash completion script from vendor/cm"

This reverts commit 734c45b20444146dc06fff66145d6ade301bbf20.