trx_toolkit: fix missing / useless imports

Found using Flake8:

  - data_if.py:57:4: F405 'log' may be undefined, or defined
                     from star imports: data_msg
  - clck_gen.py:29:1: F401 'time' imported but unused
  - clck_gen.py:30:1: F401 'sys' imported but unused
  - trx_sniff.py:28:1: F401 'signal' imported but unused

Change-Id: Id0c42319b445db218b77fd5e99a9a0a89724281d

src/target/trx_toolkit/burst_send.py

@@ -32,7 +32,6 @@ import sys
 from app_common import ApplicationBase
 from data_dump import DATADumpFile
 from data_if import DATAInterface
-from gsm_shared import *
 from data_msg import *
 
 class Application(ApplicationBase):

src/target/trx_toolkit/clck_gen.py

@@ -26,8 +26,6 @@ APP_CR_HOLDERS = [("2017-2018", "Vadim Yanitskiy <axilirator@gmail.com>")]
 
 import logging as log
 import signal
-import time
-import sys
 
 from app_common import ApplicationBase
 from threading import Timer

src/target/trx_toolkit/data_if.py

@@ -22,6 +22,8 @@
 # with this program; if not, write to the Free Software Foundation, Inc.,
 # 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
+import logging as log
+
 from udp_link import UDPLink
 from data_msg import *
 

src/target/trx_toolkit/trx_sniff.py

@@ -25,7 +25,6 @@
 APP_CR_HOLDERS = [("2018", "Vadim Yanitskiy <axilirator@gmail.com>")]
 
 import logging as log
-import signal
 import argparse
 import sys