README: remove references on no more used config CONFIG_SYS_RCAR_I2C*

Remove the references in README on CONFIG_SYS_RCAR_I2C_* not use
in U-Boot drivers/i2c/rcar_i2c.c, since commit a4d9aafa ("i2c:
 rcar_i2c: Remove the driver") and commit a06a0ac3 ("i2c: rcar_i2c:
 Add DM and DT capable I2C driver")

Checked by the command: grep -r SYS_RCAR_I2C *
And these CONFIG are only defined in
arch/arm/mach-rmobile/include/mach/rcar-base.h

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>