gdb: Remove traces of h8300 ecoff support

gdb: Remove traces of h8300 ecoff support

There's this little bit of ecoff support in the h8300 port that I ran
into that I think we could remove.  Grepping around BFD, I don't think
ECOFF is supported for h8300.

And if not, I'm failing to see why we'd support ECOFF debug info in
ELF for h8300 (.mdebug).  It kind of seems like someone just installed
the set_gdbarch_ecoff_reg_to_regnum hook at the same time the dwarf2
and stabs hooks were installed.

I'm removing this so that greps for ecoff stuff only lead to MIPS, as
ISTR that Linux/MIPS support still relies on some subset of ECOFF for
something.

gdb/ChangeLog:
2018-07-11  Pedro Alves  <palves@redhat.com>

	* h8300-tdep.c (h8300_gdbarch_init): Remove
	set_gdbarch_ecoff_reg_to_regnum calls.