Change the OS interface layer to use traditional direct function call

implementations instead of the more complicated virtual function table.
Omit the asynchronous I/O demo. (CVS 2870)

FossilOrigin-Name: 2529c2e11fa1d345ec61f647e4f6fae20a7133d6

src/os.c

@@ -69,6 +69,3 @@ int sqlite3OsLockState(OsFile *id){
 int sqlite3OsCheckReservedLock(OsFile *id){
   return id->pMethod->xCheckReservedLock(id);
 }
-struct sqlite3OsVtbl *sqlite3_os_switch(void){
-  return &sqlite3Os;
-}