clean up some target delegation cases

clean up some target delegation cases

This patch cleans up some minor inconsistencies in target delegation.
It's primary purpose is to avoid confusion in the code.  A few spots
were checking the "beneath" target; however this can only be NULL for
the dummy target, so such tests are not needed.  Some other spots were
iterating over the beneath targets, looking for a method
implementation.  This is not needed for methods handled by
make-target-delegates, as there is always an implementation.

2014-07-18  Tom Tromey  <tromey@redhat.com>

	PR gdb/17130:
	* spu-multiarch.c (spu_region_ok_for_hw_watchpoint)
	(spu_fetch_registers, spu_store_registers, spu_xfer_partial)
	(spu_search_memory, spu_mourn_inferior): Simplify delegation.
	* linux-thread-db.c (thread_db_pid_to_str): Always delegate.
	* windows-nat.c (windows_xfer_partial): Always delegate.
	* record-btrace.c (record_btrace_xfer_partial): Simplify
	delegation.
	(record_btrace_fetch_registers, record_btrace_store_registers)
	(record_btrace_prepare_to_store, record_btrace_resume)
	(record_btrace_wait, record_btrace_find_new_threads)
	(record_btrace_thread_alive): Likewise.
	* procfs.c (procfs_xfer_partial): Always delegate.
	* corelow.c (core_xfer_partial): Always delegate.
	* sol-thread.c (sol_find_new_threads): Simplify delegation.