Revert previous tweaks

Revert previous tweaks

As we migrate to the new to_xfer_partial interface, some of previous
tweaks become unnecessary, we don't have to check traceframe is
selected in each target implementation, so this patch below is
reverted.

  [PATCH] Send qXfer:traceframe-info:read when traceframe is selected.
  https://sourceware.org/ml/gdb-patches/2013-10/msg00752.html

Third, to_traceframe_info is only called when traceframe is selected,
that means it is only called when target is remote, tfile or ctf, so
this patch can be partially reverted,

  https://sourceware.org/ml/gdb-patches/2013-04/msg00000.html

gdb:

2014-02-23  Yao Qi  <yao@codesourcery.com>

	Revert two patches:

	2013-10-25  Yao Qi  <yao@codesourcery.com>

	* remote.c (remote_traceframe_info): Return early if
	traceframe is not selected.

	2013-07-19  Yao Qi  <yao@codesourcery.com>

	* target.c (update_current_target): Change the default action
	of 'to_traceframe_info' from tcomplain to return_zero.
	* target.h (struct target_ops) <to_traceframe_info>: Add more
	comments.