Simplify MortarData

[kidder]

Proposed changes

Minor cleanup of MortarData to make it easier to add an AMR projector.

• remove unused internal buffers
• remove stored TimeStepId; GTS only used them for an ASSERT, LTS already stores a TimeStepId in its BoundaryHistory
• remove insert and extract functions as they are redundant with the accessor functions

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

[kidder]

converted to draft as this will conflict with #6044 and I'll have the easier rebase

[kidder]

@knelli2 this has been rebased on develop after the merger of #6044