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.