Deprecate and remove the session argument of make()

[wlandau]

@rkrug originally raised #330 (comment) as a potential problem. By default, drake is sensitive to the environment in which make() is called. But I think we no longer need the session argument of make(). Reasons:

• make(session = callr::r_vanilla) never actually solved Creating plan in Rmd file - knit() same chunk names #330 (comment). @rkrug did make a good point that maybe objects like x should not be in the new session. But what if x is a user-defined function? For those cases, we needed x to be in the new session's global environment after all, so callr::r_vanilla() did not turn out to be useful.
• Lock the user's enviornment during make() #619 and Targets get outdated when running make() in multiple R session #615 were solved by Set lock_envir to TRUE by default #621 (make(lock_envir = TRUE)). It is still up to you to populate the environment, which admittedly requires diligence to stay consistent, but after that, make() locks the environment during runtime to prevent it from being modified.

[wlandau]

Also cc @krlmlr, @AlexAxthelm

[wlandau]

If you do not want to use the global environment, I recommend source()ing your scripts into a custom environment (new.env(parent = globalenv()) and then supplying it to the envir argument of make(). I should probably provide guidance on this in the manual.