It has never been used.

Fixes the following W=1 kernel build warning(s):

 drivers/pinctrl/meson/pinctrl-meson-a1.c:749:27: warning: ‘i2c_slave_groups’ defined but not used [-Wunused-const-variable=]
 749 | static const char const i2c_slave_groups[] = {
 | ^~~~~~~~~~~~~~~~
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Qianggui Song <qianggui.song@amlogic.com>
Cc: linux-amlogic@lists.infradead.org
Link: https://lore.kernel.org/r/20200713144930.1034632-24-lee.jones@linaro.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>