Delete ChangeLog entry with no corresponding change in sources.

Delete ChangeLog entry with no corresponding change in sources.

This ChangeLog entry was the result of a result of botched commit
which resulted in the ChangeLog update being checked in, while
the actual code code itself did not make it to CVS.

This patch deletes the entry in gdb/ChangeLog titled:

    mep: define gdbarch_pc_regnum instead of gdbarch_write_pc.