Remove TARGET_XFER_STATUS_ERROR_P

This patch removes macro TARGET_XFER_STATUS_ERROR_P, as Pedro pointed
out during patches review that TARGET_XFER_STATUS_ERROR_P tends to
be unnecessary.

gdb:

2014-02-24  Yao Qi  <yao@codesourcery.com>

	* target.h (TARGET_XFER_STATUS_ERROR_P): Remove.
	* corefile.c (read_memory): Adjusted.
	* target.c (target_write_with_progress): Adjusted.