Kconfig: Drop CONFIG_CMD_PCI_ENUM

This option enables the 'pci enum' command. It is only enabled by a few
board and these have not yet been converted to driver model, which always
enables this command. It seems easiest to just remove this option.

The affected boards can be converted to use driver model for PCI if
needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>