Remove smmaco4 board support

Recent changes caused that the smmaco4 board now is included in the
boards built by MAKEALL, which revealed that compilation for this
board has been broken for a long time:

mpc5xxx_fec.c:899:2: error: #error fec->xcv_type not initialized.

The board is orphaned.  Drop support for it.

Signed-off-by: Wolfgang Denk <wd@denx.de>