update documentation and tooling w.r.t. *.env file setup

[das-g]

It seems the need for *.env files has been removed with PR #884, specifically with commit 0d957f9.

If so, the make target local_dev_env (which currently does nothing except for printing a message) should be removed and the README.md updated accordingly (which probably means that subsection "Initialization" of section "Run it locally on Linux (Development)" should simply be removed.)

(As far as I can see, other documentation isn't affected as it doesn't cover this.)