Remove TensorProductElementGroupBase.bases_1d

[inducer]

To get the tests in inducer/grudge#338 to go, zero-dimensional tensor product discretizations need to work. This bases_1d thing (which seems kind of ill-defined anyway) was an obstacle. It appeared entirely unused across meshmode and grudge.

@a-alveyblanc Do you have any objections to this?

[a-alveyblanc]

This is necessary to get the 1D basis (bases) used to build the TP basis so that we can retrieve 1D operators.

inducer/grudge#313 depends on bases_1d for fast operator evaluation to work

(sorry if formatting is bad, I'm replying on GH mobile)

EDIT: From what I can tell by the changes, this can be worked around in inducer/grudge#313. Seems fine to merge.

[inducer]

OK, thanks!