z88cyrus.asm: Remove "ZX" compilation specific IF blocks and various dead code that is no longer used

z88cyrus.asm is the z88-specific implementation, copied from original cyrus.asm (the original ZX Spectrum reverse-engineered source code).

Remove all IF ZX - ENDIF conditional compilation blocks as first step. Then, gradually remove dead code from functionalities no longer used, optionally evaluating it for preservation for future use or not.