Remove unused labels in dwarf assembler

I happen to see that 'double_label' isn't used in dwz.exp dwarf assembler.
Similarly, partial_label and double_label aren't used in dwzbuildid.exp.
This patch is to remove them.

gdb/testsuite:

2014-04-25  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/dwz.exp (Dwarf::assemble): Remove unused
	double_label.
	* gdb.dwarf2/dwzbuildid.exp (Dwarf::assemble): Remove
	partial_label and double_label.