Remove misleading comment

The SIMD matrix equivalency with GL is actually confusing readers.

The origin of the comment can be traced in the 3D matrix type inside
Gecko:

> This matrix is treated as row-major. Assuming we consider our vectors
> row vectors, this matrix type will be identical in memory to the
> OpenGL and D3D matrices. OpenGL matrices are column-major, however
> OpenGL also treats vectors as column vectors, the double transposition
> makes everything work out nicely.

Graphene uses the same strategy of using row vectors, given that we are
operating on them through SIMD instructions.