Fix leak of auto_obstack objfile_per_bfd_storage->storage_obstack;

Fix leak of auto_obstack objfile_per_bfd_storage->storage_obstack;

  commit 23732b1e32dd58f7c731d9aee56ff0b22a645d53
  Author: Pedro Alves <palves@redhat.com>
  Date:   Tue Jun 27 16:22:08 2017 +0100
changed objfile_per_bfd_storage->storage_obstack
from  'struct obstack storage_obstack;'
to    'auto_obstack storage_obstack;'
So the obstack is auto allocated when the  objfile_per_bfd_storage ctor is
manually called by get_objfile_bfd_data).
However, the ctor call was still followed by a manual call to
      obstack_init (&storage->storage_obstack);

This results in a bunch of leaks detected by valgrind, such as:
==24665== 4,064 bytes in 1 blocks are definitely lost in loss record 11,469 of 11,590
==24665==    at 0x4C27BF5: malloc (vg_replace_malloc.c:299)
==24665==    by 0x5437B7: xmalloc (common-utils.c:44)
==24665==    by 0x77CAA7: _obstack_begin_worker (obstack.c:141)
==24665==    by 0x60168F: auto_obstack (gdb_obstack.h:70)
==24665==    by 0x60168F: get_objfile_bfd_data(objfile*, bfd*) (objfiles.h:188)
==24665==    by 0x601DB6: allocate_objfile(bfd*, char const*, enum_flags<objfile_flag>) (objfiles.c:423)
==24665==    by 0x647753: symbol_file_add_with_addrs(bfd*, char const*, enum_flags<symfile_add_flag>, section_addr_info*, enum_flags<objfile_flag>, objfile*) (symfile.c:1158)
==24665==    by 0x647C7B: symbol_file_add_separate(bfd*, char const*, enum_flags<symfile_add_flag>, objfile*) (symfile.c:1252)
==24665==    by 0x4C7D79: elf_symfile_read(objfile*, enum_flags<symfile_add_flag>) (elfread.c:1270)
==24665==    by 0x647CB4: read_symbols(objfile*, enum_flags<symfile_add_flag>) (symfile.c:861)
==24665==    by 0x647809: syms_from_objfile_1 (symfile.c:1062)

-> remove the manual call to obstack_init.
Reg-tested on Debian 8/amd64, tests results are the same before/after the patch.
valgrind still show some leaks, but less.

gdb/ChangeLog
2017-09-05  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* objfiles.c (get_objfile_bfd_data): Remove useless obstack_init
	call.