Read first and heed the note in my last line...
If you drill down the filing tree until you reach the folder with the aircraft.cfg in it, you'll also see a folder named "panel". Open it.
(hint: F:\msfs\Official\OneStore\asobo-aircraft-c172sp-classic\SimObjects\Airplanes\Asobo_C172sp_classic) - hopefully though, you'll be doing this on a repaint in the community folder.
You'll see a file panel.cfg - open it in notepad.
At the bottom of the text you will see:
[VPainting01]
size_mm = 256, 64
texture = $RegistrationNumber
location = interior
painting00=Registration/Registration.html?font_color=white, 0, 0, 256, 64
[VPainting02]
size_mm = 1024, 256
texture = $RegistrationNumber
location = exterior
painting00=Registration/Registration.html?font_color=0x49668e, 0, 0, 1024, 256
Obviously VPainting01 places the tail number in your cockpit and VPainting02 is that white patch in your screenshot. From what little I understand the two numbers in size_mm = 256, 64 are the box size. and the group of four below affect the text start position and size... 0, 0, places the text top left start position and 1024, 256 the bottom right end pos.
Now you can change all those numbers in both blocks to 0 and the white blocks in your cockpit and on the side of the fuselage will disappear.
It's not a perfect solution, I know, but it works. And of course: DON'T FORGET TO MAKE A BACKUP OUTSIDE OF THE GAME FOLDERS