Remove cleanups from mi-cmd-var.c

Remove cleanups from mi-cmd-var.c

This removes some cleanups from mi-cmd-var.c.  varobj_gen_name now
returns a string, simplifying mi_cmd_var_create.  In
mi_cmd_var_delete, a string copy is apparently unnecessary, so it's
simply removed.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* varobj.h (varobj_gen_name): Return std::string.
	* varobj.c (varobj_gen_name): Return std::string.
	* mi/mi-cmd-var.c (mi_cmd_var_create): Use std::string.
	(mi_cmd_var_delete): Don't copy "name".