Improve Tails Cloner info bar and delete message text

"Please plug a USB flash drive" is weird language, we use the phrase "plug in a USB stick" or "plug a USB stick into" elsewhere.

It's easier to see the command without "Please" in front and matches style guide:

• please

  Avoid please except in situations where the user is asked to do something inconvenient or the software is to blame for the situation.

I don't consider plugging in a USB stick inconvenient.

"USB flash drive" needs to change to match our style as well:

• USB stick

  And not USB drive, USB, thumb drive, or flash drive.

Before:

"Plug in a USB stick"