fix error in nios2-tdep.c

fix error in nios2-tdep.c

Since this change:

2014-02-12  Sanimir Agovic  <sanimir.agovic@intel.com>

	* nios2-tdep.c (nios2_stub_frame_base): Remove global.

nios2-tdep hasn't built:

../../binutils-gdb/gdb/nios2-tdep.c:1337:1: error: ‘nios2_stub_frame_base_address’ defined but not used [-Werror=unused-function]

This patch removes the offending function.

2014-02-12  Tom Tromey  <tromey@redhat.com>

	* nios2-tdep.c (nios2_stub_frame_base_address): Remove.