remove mmtf in 3.0

[orbeckst]

Remove all mmtf-handling code for 3.0.

• MMTF is no longer supported by PDB (see #4634 )
• the mmtf-python license is incompatible with our target license LGPLv2.1+ (see https://www.mdanalysis.org/2023/09/22/licensing-update/#why-the-interim-gpl-v3-and-lgpl-v3-package-licenses-the-issue-with-apache-20 )