configs/freescale_imx31_3stack: Remove target

configs/freescale_imx31_3stack: Remove target

Currently this target fails to build:

arch/arm/mach-imx/built-in.o: In function `init_mmdc_lpddr2_settings':
platform-spi_imx.c:(.text+0x8f18): undefined reference to `imx6sll_lpddr2_freq_change'
arch/arm/mach-imx/built-in.o: In function `imx6ul_cpuidle_init':
platform-spi_imx.c:(.init.text+0x195c): undefined reference to `mx6ull_lpm_wfi_end'
platform-spi_imx.c:(.init.text+0x1960): undefined reference to `mx6ull_lpm_wfi_start'
platform-spi_imx.c:(.init.text+0x1964): undefined reference to `imx6ull_low_power_idle'
arch/arm/mach-imx/built-in.o: In function `imx6sl_init_late':
platform-spi_imx.c:(.init.text+0x56f0): undefined reference to `imx6sll_cpuidle_init'
Makefile:969: recipe for target 'vmlinux' failed

NXP 4.9 kernel does not properly support mx31, so better remove this target.

mx31_3stack board is currently supported in U-Boot and kernel mainline,
so if someone is interested on this board, then a target can be introduced
later which uses these mainline components.

Currently I do not have access to this board, so remove it its defconfig
for now.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/66561688
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>