Revert "box: fix shared lang between connected clients"

This reverts commit 8aa2bb19.

In scope of the aforementioned commit changes from commit e4fda4b7
("box: fix shared lang between connected clients") are backported, but
the test in this patch uses test.interactive_tarantool.lua helper, that
is missing in release/2.10 branch. Applying all the related commits from
the master listed below doesn't fix the problem:
* commit a9d96007 ("test: add a helper for testing interactive
  mode")
* commit 5493db7c ("test: mv interactive_tarantool.lua to ./test/
  dir")
* commit ed86a729 ("test: add prompt setter to interactive helper")

Hence, simply revert the poisoned commit from the branch.

NO_DOC=revert
NO_TEST=revert
NO_CHANGELOG=revert