Remove pcfsoft shadow type, deprecated in r182, pcf is now soft as well

[vincentfretin]

Description:

Remove pcfsoft shadow type, deprecated in r182, pcf is now soft as well.
You should wait for three to be updated to r182+ before merging this.

Changes proposed:

• Remove pcfsoft from the available shadow types
• Change pcfsoft to pcf in the docs and examples
• Add migration guide in changelog

[mrxz]

Perhaps we can add a generic fallback in shadow#init for when the given shadow type isn't known. That should more gracefully handle any mistyped/unknown types and has the added benefit of not breaking when users upgrade, while a warning in the console can still clue them in to change/fix it.

[vincentfretin]

Right, the warning on threejs side THREE.WebGLShadowMap: PCFSoftShadowMap has been deprecated. Using PCFShadowMap instead..
I propose shadow type "pcfsoft" is not supported, falling back to "pcf". To remove this warning set <a-scene shadow="type: pcf">.
I did the change.