Remove omniauth-facebook

[DanielePalombo]

omniauth-facebook extension downgrades omniauth lesser than V2. Rather than lock omniauth version, we think it would be better to remove omniauth-facebook gem and the commit that lock the omniauth version so that the user can add it back manually if required.

ref #102 (comment)
9bc5e05

[piyushswain]

On further investigation, the problem now seems to be with the omniauth gem instead of the omniauth-facebook gem.
Some breaking changes were introduced in omniauth for v2.0 (changelog) which have resulted in failing sign_in_specs.

This issue appears only in omniauth versions >= 2.0. Solidus Social is still working fine for omniauth version 1.9.1.

[piyushswain]

This is the error when clicking on a social login option on the login page