gdb manual: Remove references to deleted targets

gdb manual: Remove references to deleted targets

Support for target dbug/picobug/dink32/m32r/mon2000/ppcbug was just
removed, but support for ARM RDI, Sparclet, Sparclite, Z8000, target
r3900, target array, target sds, target op50n and target w89k had
already been removed many years ago.  Drop it all in one go.

gdb/doc/ChangeLog:
2015-08-24  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Embedded Processors) <ARM>: Remove references to RDI.
	<M32R>: Remove references to M32R/D.
	<M68K>: Remove references to target dbug.
	<MIPS Embedded>: Remove references to target r3900 and target
	array.
	<PowerPC Embedded>: Remove references to target dink32 and target
	ppcbug, target sds
	<PA, Sparclet, Sparclite, Z8000>: Delete nodes.