Drop `:=` for S7 compatibility

[hadley]

RConsortium/S7#697

Looks like maybe you're working on a release at the moment? It would be great to squeeze this in there if possible.

[lionel-]

Unfortunately we can't remove it from rlang without causing NOTEs in downstream packages:

* checking R code for possible problems ... NOTE
f: no visible global function definition for ':='
Undefined global functions or variables:
  :=