Remove old fast scroll routines from CBM-II version

Revert "Add fast scroll support for 80x25 screen"
This reverts commit 9c4775e17275965eb3df695f532d4936ecc7aa77.

Revert "Use an immediate value to erase the last line"
This reverts commit a1550b2cbdced70fdefda388417cb7aa14608e94.

Revert "Implement fast scroll for 80x24 screen"
This reverts commit 94d77ef4db1d4831f5c512568eef31fedcfdbdf2.