board: toradex: verdin-am62: Remove not needed env variables

Remove not needed variables from environment.

 - boot_scripts is not needed, the default value is just fine and
   already includes boot.scr
 - setup variable used to be executed from some bootscript, however
   it's not required and there is no point on having this small helper
   here

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>