configs: airoha: an7581: disable ENV_IS_IN_MTD to avoid boot panic

configs: airoha: an7581: disable ENV_IS_IN_MTD to avoid boot panic

Booting image generated with

  make an7581_evb_defconfig

will results in

  U-Boot 2026.04-00924-gfb815bd8793b (Apr 27 2026 - 15:08:30 +0300)

  CPU:   Airoha AN7581
  DRAM:  512 MiB
  Core:  35 devices, 19 uclasses, devicetree: separate
  MMC:   mmc@1fa0e000: 0
  Loading Environment from MMC... *** Warning - No block device, using default environment

  Loading Environment from MTD... *** Warning - get_mtd_device_nm() failed, using default environment

  BUG at drivers/mtd/mtdcore.c:898/__put_mtd_device()!
  BUG!
  resetting ...

This happens because no any mtd partition defined in dts/mtdparts.
Disabling of ENV_IS_IN_MTD fixes an issue.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>