Revert "Update prebuilts to include 2.0.0-beta6"

This reverts commit 750b8cbdbd3dc20471e3381fd036d374d21b66d6.

The original CL removed beta5 prebuilts and added beta6. Instead of
removing beta5, it is preferable to remove an older beta, as removing
beta5 means that users will experience a plugin download of beta5 during
the first gradle sync after which they will be offered the beta6 update.
75 files changed