Remove policies support from the IDE

[Therzok]

We should try and figure out if we really want to keep policy support for 8.0 onward.

Personally, it's unpredictable in behaviour, as policy settings take place if no equivalent editorconfig setting is active.

cc @mhutch @mkrueger @slluis

VS bug #730265

[Therzok]

As to not break users, we could do the following:

• Move policy reading/writing support into a separate addin. Detecting policies being used would offer to migrate to editorconfig.
• The policy editing UI could probably just surface editorconfig file.

[mhutch]

See https://github.com/mono/monodevelop/projects/3

I would argue that we need a good editorconfig editing experience before deprecating policies.