[mlir][Linalg] Drop function attribute from generic ops.

The function attribute in generic ops is not paying for itself.
A region is the more standardized way of specifying a custom computation.
If needed this region can call a function directly.
This is deemed more natural than managing a dedicated function attribute.

This also simplifies named ops generation by trimming unnecessary complexity.

Differential Revision: https://reviews.llvm.org/D78266