Remove explicit tail-call support

[chklauser]

Summary

• remove the call\\tail macro and call\\tail\\perform runtime command
• remove the dedicated tail bytecode instruction and VM/CIL/parser handling
• remove the obsolete exports and implementation-specific tests
• retain compiler-managed self-tail-recursion optimization, all return modes, and call\\sub

Validation

• dotnet test --no-restore (1,432 total; 1,429 passed; 3 skipped)