Yank out release tagging and (clien-side) version checking support.

(This is now obsolete, since the hybrid extension packaging is gone.)

This reverts the following commits:
    0651af1f1806a76efebe7bb3ea55499c11f07787
    7fbe0a71e2403ee426b311f24c4d27e086025c76
    192b9547528541c03945975a42d65c1c75d4cd65