Remove rename_targets()

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bug still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Description

Unfortunately, we cannot (at least, we should not) rename targets. We need to remove rename_targets() from drake.

Each target comes with its own random number generator seed, which is determined by the global seed (seed argument of make()) and the target name. There is simply no other way to generate a different reproducible RNG seed for each target. By renaming a target, we are changing the seed the target was supposed to have, which invalidates the value in the cache.