gdb: remove unused includes from dwarf2read.c

gdb: remove unused includes from dwarf2read.c

include-what-you-use says:

../../../src/binutils-gdb/gdb/dwarf2read.c should remove these lines:
- #include <ctype.h>  // lines 67-67
- #include <sys/stat.h>  // lines 59-59
- #include <sys/types.h>  // lines 83-83
- #include <cmath>  // lines 88-88
- #include <forward_list>  // lines 90-90
- #include <set>  // lines 89-89
- #include <unordered_set>  // lines 85-85
- #include "completer.h"  // lines 60-60
- #include "expression.h"  // lines 44-44
- #include "gdbsupport/byte-vector.h"  // lines 78-78
- #include "gdbsupport/filestuff.h"  // lines 71-71
- #include "gdbsupport/gdb_unlinker.h"  // lines 74-74

After a quick glance, that makes sense, so this patch removes them.

gdb/ChangeLog:

	* dwarf2read.c: Remove includes.

Change-Id: I13cfcb2f1d747144fddba7f66b329630b79dae90