Remove alignmentscope from MathML 4 spec

[NSoiffer]

alignmentscope is a MathML 3 mtable attribute that is part of the complicated spec for maligngroup/malignmark. Those elements have been simplified considerably for MathML 4, and alignmentscope is no longer part of how they function. However, it is still in the spec and needs to be removed.

[davidcarlisle]

Note to self to move the attribute to the legacy schema at same time.
The removal from the spec should probably be part of PR #532