Remove xtensor

[garth-wells]

This change removes xtensor. xtensor was used only in the C++ interface for mesh entity marking and user expressions. Other internal use was removed earlier.

Removing xtensor

• reduces the number of dependencies
• avoids ABI incompatibilities that can sneak in if xsimd is installed
• avoids potential C++20 compatibility issues (xtensor does not support C++20)

Closes #2350.

[francesco-ballarin]

Is it correct to say that after this PR is merged none of the packages in the FEniCSx ecosystem will require xtl or xtensor?

If so, people like @drew-parsons and myself should make sure to remove such dependency in their packaging workflows.

[garth-wells]

Is it correct to say that after this PR is merged none of the packages in the FEniCSx ecosystem will require xtl or xtensor?

Yes, correct.