m68k: Drop unreferenced CONFIG_* defines

The following config symbols are only defined once and never referenced
anywhere else:

CONFIG_AMCORE
CONFIG_ASTRO5373L
CONFIG_M52277EVB
CONFIG_M5253DEMO
CONFIG_M5253EVBE
CONFIG_M5275EVB
CONFIG_M54418TWR
CONFIG_STMARK2

Most of them are config symbols named after the respective boards which
seems to have been a standard practice at some point.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>