board/stm32f769-disco: removed unknown command line parameters in extlinux.conf. ...

The kernel doesn't know about consoleblank nor earlyprintk, so remove them both. They were uselessly passed to pid 1.

Initial commit for stm32f769-disco: ...

* Buildroot version 2021.02.7.
* Das U-Boot version 2021.10.
* Linux kernel version 5.15.3.
* Busybox 1.33.1.