Remove read_memory_string

Remove read_memory_string

read_memory_string is redundant and only called in a couple of spots.
This patch removes it in favor of target_read_string.

gdb/ChangeLog
2020-06-15  Tom Tromey  <tromey@adacore.com>

	* corefile.c (read_memory_string): Remove.
	* ada-valprint.c (ada_value_print_ptr): Update.
	* ada-lang.h (ada_tag_name): Change return type.
	* ada-lang.c (type_from_tag): Update.
	(ada_tag_name_from_tsd): Change return type.  Use
	target_read_string.
	(ada_tag_name): Likewise.
	* gdbcore.h (read_memory_string): Don't declare.