Consider what to do with date+zone semantic skeletons

[sffc]

Related: https://github.com/unicode-org/icu4x/issues/6690

The ICU TC recommended that CLDR consider removing date+zone from semantic skeletons. https://unicode-org.atlassian.net/browse/ICU-23233

We currently support this in 2.0, and I don't anticipate removing it from ICU4X in the short term. We should re-evaluate in 3.0.

CLDR ticket about removing this from semantic skeletons: https://unicode-org.atlassian.net/browse/CLDR-19066