refactor: remove cpo-& behavior (#17745)

cpo-& has been removed, but its behavior was accidentally made the
default behavior. That should be removed instead.

src/nvim/buffer_defs.h

@@ -90,7 +90,6 @@ typedef struct {
 #define BF_NEW_W        0x20    // Warned for BF_NEW and file created
 #define BF_READERR      0x40    // got errors while reading the file
 #define BF_DUMMY        0x80    // dummy buffer, only used internally
-#define BF_PRESERVED    0x100   // ":preserve" was used
 #define BF_SYN_SET      0x200   // 'syntax' option was set
 
 // Mask to check for flags that prevent normal writing