* configure.in: Eliminated all targets using obj-coff but not defining

* configure.in: Eliminated all targets using obj-coff but not defining
BFD_ASSEMBLER; I think all such targets that are supported will be matched by
real CPU-OS combinations earlier in the case statement.

(targets *-*-coff*, *-sysv*, *-*-sco*, *-*-sysv32): Deleted.  Made some
comments about the dpx2 configuration, but left it disabled, since it couldn't
be reached before.
(target a29k-amd-ebmonold): Deleted.