README: i2c: Drop unused i2c CONFIG options

CONFIG_SYS_NUM_I2C_ADAPTERS and CONFIG_SYS_I2C_MULTI_NOPROBES are not used
in U-Boot, so drop them.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>