Remove unused imports from gdb's Python code

The "sys" import is unused in several Python files.  This removes this
line from all the places where it is unnecessary.

gdb/python/lib/gdb/command/explore.py

@@ -17,7 +17,6 @@
 """Implementation of the GDB 'explore' command using the GDB Python API."""
 
 import gdb
-import sys
 
 
 class Explorer(object):

gdb/python/lib/gdb/printer/bound_registers.py

@@ -14,8 +14,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import sys
-
 import gdb.printing
 
 

gdb/python/lib/gdb/printing.py

@@ -19,7 +19,6 @@
 import gdb
 import gdb.types
 import re
-import sys
 
 
 class PrettyPrinter(object):

gdb/python/lib/gdb/xmethod.py

@@ -18,7 +18,6 @@
 
 import gdb
 import re
-import sys
 
 
 class XMethod(object):