i2c: Drop CONFIG_SH_SH7734_I2C

Last user of this driver went away in May 2017 in commit
eb5ba3ae ("i2c: Drop use of CONFIG_I2C_HARD").

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Acked-by: Heiko Schocher <hs@denx.de>