[BOOTSCRIPT] Remove old custom boot.env file of EVK machines

This because we are goint to use a new bootscript management where a set
of placeholders is used to parametrize the various boot*.scr and
boot*.env files. In this way (only for the actual stage of the
development) is no longer need to have custom script per machine and a
set of flags of the variable BOOTSCRIPT_PARAM (see documentation and
implementation in meta-seco-bsp) will be added to machine related file
(seco-imx8-common.inc, seco-imx93-common.inc, ...).
The customization can be performet at both SOC and board/SOM levels.

Signed-off-by: Davide Cardillo <davide.cardillo@seco.com>

recipes-bsp/bootscript/bootscript.bbappend

-FILESEXTRAPATHS:prepend := "${THISDIR}/files:"

recipes-bsp/bootscript/files/nxp-imx8mpevk/boot.env

-kernel=Image
-devicetree=imx8mp-evk.dtb
-cmdline="console=ttymxc1,115200"
-boot_part=1
-root_part=2
-

recipes-bsp/bootscript/files/nxp-imx93evk/boot.env

-kernel=Image
-devicetree=imx93-11x11-evk-pmic-pf0900.dtb
-cmdline="console=ttyLP0,115200"
-boot_part=1
-root_part=2

recipes-bsp/bootscript/files/nxp-imx95evk/boot.env

-kernel=Image
-devicetree=imx95-19x19-evk.dtb
-cmdline="console=ttyLP0,115200"
-boot_part=1
-root_part=2