sbc8349: Remove SDRAM functionality

The MPC8349EMDS configuration was the basis for the sbc8349, so it also
contains its SDRAM option.

Since
* the SDRAM has to be soldered onto the board,
* the sbc8349 never used the support, and
* the support never worked (see previous patch fixing it),

we can assume that the support on the sbc8349 is an artifact created by
copying the MPC8349EMDS config wholesome.

Hence, instead of creating a separate sbc8349 config that supports
SDRAM, we can remove the SDRAM option for this board.

Should it be needed in the future, it can be copied from the new
MPC8349EMDS_SDRAM board.

Signed-off-by: Mario Six <mario.six@gdsys.cc>