Replace VEC(converted_character_d) with std::vector

Replace VEC(converted_character_d) with std::vector

This patch changes the usage of VEC(converted_character_d) to use an
std::vector instead.  This allows getting rid of a cleanup.

gdb/ChangeLog:

	* valprint.c (converted_character_d): Remove typedef.
	(DEF_VEC_O (converted_character_d)): Remove.
	(count_next_character): Use std::vector.
	(print_converted_chars_to_obstack): Likewise.
	(generic_printstr): Likewise.