I installed grub twice on two drives, how to fix it?

[M. Fioretti]

Greetings,

Last month, I bought a used Lenovo T430 laptop with Windows on it and,
since I needed Linux on it in a hurry, I installed Ubuntu 19.10 on it,
alongside Windows, on its factory-installed hard drive.

Last week I finally had the time to mount an MSATA SSD drive into the
laptop. The plan, taking into account advice received here on the list
in previous threads, was/is to:

1) install Ubuntu on MSATA/SSD Drive, with swap on it too

2) keep windows and /home on previous hard drive

3) as soon as I am sure I can get rid of windows for good (not now,
   alas...) erase its partition, use that whole hard drive for /home

so I can keep system+boot info + swap on SSD and data on hard drive,
totally separated

The problem is that during the installation I got distracted, and now
I have (do not remember how/when/why, honestly) grub installed TWICE,
both on old hard drive /dev/sda, and on SSD drive, /dev/sdb, pointing
to two distinct Ubuntu installs, one per drive. For details, please
see full output of bootinfoscript at
http://zona-m.net/marco/bootscriptinfo.txt

In practice, now when I boot I get a grub menu giving me 3 choices
(besides memtest, and safe/recovery options), in this order, all
working:

* Ubuntu 19.10 on /dev/sda (old hard drive)
* windows
* Ubuntu 19.10 on /dev/sdb (new SSD drive)

I want, instead, to have only one grub only on MBR of the SSD drive
(/dev/sdb), so the system boots even if I replace the hard drive
(/dev/sda), and offering only two options, Ubuntu on ssd drive, and
windows, until I erase it.

What is the best way to fix this mistake I made?

Thanks,
Marco