i2c: Remove CFG_I2C_MULTI_BUS

This is used by a few boards but we are years past the migration date,
so let's drop it now.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/include/i2c.h b/include/i2c.h
index 1d3be3f..622540c 100644
--- a/include/i2c.h
+++ b/include/i2c.h
@@ -942,7 +942,7 @@
  * only for backwardcompatibility, should go away if we switched
  * completely to new multibus support.
  */
-#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY) || defined(CFG_I2C_MULTI_BUS)
+#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 # if !defined(CFG_SYS_MAX_I2C_BUS)
 #  define CFG_SYS_MAX_I2C_BUS		2
 # endif