Possibly remove <<- from the default valid operators in assignment_linter

[MichaelChirico]

Follow-up to #2711. I think the messaging and signature of assignment_linter() will be cleaner if there's only one valid operator by default.

I also think it's worth noting the conflict between the undesirable_operator_linter() defaults and the assignment_linter() defaults.

Filing this issue for now so as not to introduce one more breaking change in the current release.

Upstream issue to watch:

tidyverse/style#239