stm32mp: remove the bootcount activation

stm32mp: remove the bootcount activation

Today the bootcount is not managed by the Linux kernel for STM32MP15 as
we don't have driver to update the used backup register in TAMP and the
recovery command still executes the normal bootcmd with
'altbootcmd=run bootcmd'.

So the bootcount feature is never used, the config CONFIG_BOOTCOUNT_LIMIT
and the associated environment variable 'altbootcmd' can be removed to
reduce the U-Boot size.

Each boards can re-enable this feature later in their defconfig, if it is
needed, with the expected backend, for example CONFIG_BOOTCOUNT_GENERIC
or CONFIG_BOOTCOUNT_ENV.

CC: Marek Vasut <marex@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>