Fix corrupt DWARF in dw2-double-set-die-type

Fix corrupt DWARF in dw2-double-set-die-type

The dw2-double-set-die-type.exp test case caused an AddressSanitizer
failure in the new DWARF scanner.

The immediate cause was bad DWARF in the test -- in particular, the
the sibling attribute here:

     <2><181>: Abbrev Number: 33 (DW_TAG_subprogram)
	<182>   DW_AT_external    : 1
	<183>   DW_AT_name        : address
	<18b>   DW_AT_type        : <0x171>
	<18f>   DW_AT_declaration : 1
	<190>   DW_AT_sibling     : <0x1a1>
    ...
     <1><1a1>: Abbrev Number: 23 (DW_TAG_pointer_type)
	<1a2>   DW_AT_byte_size   : 4
	<1a3>   DW_AT_type        : <0x1a7>

...points to a "sibling" DIE that is at a different child depth.

Because this test case doesn't really require sibling attributes, this
patch fixes the problem by removing them from the test.

Note that gdb is not generally robust against malformed DWARF.
Detecting and compensating for this problem would probably be
expensive and, IMO, is better left to some (still hypothetical) DWARF
linter.