Remove unused imports

I noticed an unused import in dap/evaluate.py; and also I found out
that my recent changes to use frame filters from DAP left some unused
imports in dap/bt.py.

gdb/python/lib/gdb/dap/bt.py

@@ -15,11 +15,8 @@
 
 import gdb
 import os
-import itertools
 
 from gdb.frames import frame_iterator
-from gdb.FrameIterator import FrameIterator
-from gdb.FrameDecorator import FrameDecorator
 from .frames import frame_id
 from .server import request, capability
 from .startup import send_gdb_with_response, in_gdb_thread

gdb/python/lib/gdb/dap/evaluate.py

@@ -14,7 +14,6 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import gdb
-import gdb.printing
 
 # This is deprecated in 3.9, but required in older versions.
 from typing import Optional