MAINT: remove preference of lalframe.

This MR removes a hardcoded preference for lalframe. I looked back through the history and it isn't clear why we set this as the default (it appeared in this commit).

The main issue with this method is that it cannot read some frames that are not a power of 2 in length which causes failures at the edges of observing segments. GWpy has a preferred order. I think we should just go with this. It's possible the original motivation was to avoid the conda dependency on framecpp, but I think we can live with that. Also, given the recent changes to the data generation, this gwpy.get branch is no longer a preferred method.

If we really want this to be the default, we should set it in the parser.