gdb: no longer define __STDC_CONSTANT_MACROS/__STDC_LIMIT_MACROS

gdb: no longer define __STDC_CONSTANT_MACROS/__STDC_LIMIT_MACROS

My gnulib fix at:
  https://lists.gnu.org/archive/html/bug-gnulib/2015-11/msg00010.html
was merged upstream meanwhile and our gnulib copy now includes it.

As a concidence, Kevin was telling me today that these macros are
causing a build problem on FreeBSD:

common/common-defs.h:47:0: error: "__STDC_CONSTANT_MACROS" redefined [-Werror]
  #define __STDC_CONSTANT_MACROS 1
  /usr/include/sys/cdefs.h:408:0: note: this is the location of the previous definition
  #define __STDC_CONSTANT_MACROS
(and a similar error for __STDC_LIMIT_MACROS)

The problem seems to be that we should be defining these input macros
before including any system header, but, we're not.

So let's just revert e063da67902e ([C++] Define __STDC_CONSTANT_MACROS
/ __STDC_LIMIT_MACROS for stdint.h).  If this causes a problem
somewhere, we can re-define the macros higher up in the file, before
system headers are included.

gdb/ChangeLog:
2016-10-18  Pedro Alves  <palves@redhat.com>

	* common/common-defs.h (__STDC_CONSTANT_MACROS)
	(__STDC_LIMIT_MACROS): Delete.