Consider deprecating/removing sops exec --background flag

[ajvb]

Initially raised by @alx13 in https://github.com/mozilla/sops/pull/627#issuecomment-584328589

The --background flag for the sops exec-env and sops exec-file is not particularly useful and backgrounding commands should really be done using your shell environment.

[felixfontein]

I created #1379 for deprecating --background.