/sdf/world/physics/@type attribute specification contains Gazebo Classic-specific details.

While reviewing https://github.com/robotology/gym-ignition/pull/336/files, I noticed that `ignored` is not an allowed value for the `/sdf/world/physics/@type` attribute according to the SDF spec: http://sdformat.org/spec?ver=1.8&elem=physics#physics_type .

However, the problem seems to be that the /sdf/world/physics/@type attribute specification contains Gazebo Classic-specific aspects.

## Environment
Issue related to SDF specitication, version 1.8

## Description
* Expected behavior: The SDFormat specification, being used in different simulator, should not contain Gazebo Classic information that do not apply to other simulators (such as Ignition Gazebo)
* Actual behavior: The list of supported physics engine in the SDF format specification is the one of Gazebo Classic.

## Steps to reproduce
Read http://sdformat.org/spec?ver=1.8&elem=physics#physics_type .

This is related to https://github.com/osrf/sdformat/issues/508, but it is specific about the `/sdf/world/physics/@type` attribute while that issue is more related to the `/sdf/world/physics/ode`, `/sdf/world/physics/bullet` elements.

Start with the SDF 1.8 physics specification at the linked /sdf/world/physics/@type entry and compare its supported values with the simulator-neutral behavior described in this issue. Locate the specification source for that attribute and update it so Gazebo Classic-specific details do not constrain other simulators; the linked entry should then reflect the corrected values.

Written by the indexing model from the issue text.