In 1.12.4, Private Division demonstrated their unwillingness to learn from other game devs (looking at you, paradox) that players don't want their ♥♥♥♥♥♥♥ bloatware launchers.

Unfortunately, reverting back to 1.12.3 does not fix this. 1.12.4 is an unneeded update that only fixes tutorials and adds the launcher. This launcher still installs if you revert to an earlier version.

STEP 1
Go to your KSP folder.

STEP 2
Right click on KSP_x64.exe
Click "Create shortcut"

STEP 3
Move the shortcut to your desktop.
OPTIONAL: Rename it to KSP or Kerbal Space Program.

STEP 4
Go to launch options in KSP's properties window and put in
"[KSP install location]\KSP_x64.exe" %command% -popupwindow

STEP 5
Done.