BREAK:remove unconditional 'are you sure?' prompts

This is a breaking change, as it affects behaviour that people might be
relying on. An entry has been added to NEWS.

As proposed on the mailing list, this change removes the unconditional
prompts on:

    o: bst workspace reset
    o: bst workspace close --remove-dir

If interactive, these commands would always interrupt you with a prompt
like this:

    This will remove all your changes, are you sure?

This seems like it may just save someone's work some time. It may also
condition folks to hit 'y' quickly without thinking.

This change also makes the non-interactive behaviour consistent with the
interactive behaviour in the default case. There is also the case of the
prompt configured by 'really-workspace-close-project-inaccessible',
which may be tackled in later work.

This change also removes the new config options to suppress those
prompts, and their associated news entry.

The relevant bit of the mailing list conversation is here:
https://mail.gnome.org/archives/buildstream-list/2018-December/msg00106.html

The issue to make interactive and non-interactive behaviour consistent
is here:
#744