config: stm32mp15: remove CONFIG_FASTBOOT_CMD_OEM_FORMAT

Remove the support of the fastboot "oem format" command for STM32MP15x
boards and removed the associated env variable "partitions".
This command is not required; with fastboot tool, the GPT partition can
be handle with "flash" command in "gpt" target (=CONFIG_FASTBOOT_GPT_NAME),
for example: fastboot flash gpt gpt.bin

This patch avoids to define the GPT partitioning in U-Boot environment,
which is incompatible with planned modifications, for example to
support TF-A firmware update.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>