gdb_readline -> gdb_readline_no_editing

gdb_readline -> gdb_readline_no_editing

Name this such that it's clearer that this is not a wrapper for the
real readline, but instead a replacement that provides no command line
editing features.

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* defs.h (gdb_readline): Delete declaration.
	* top.c (gdb_readline): Rename to ...
	(gdb_readline_no_editing): ... this, and make static.