Drop event_loop global functions and import MainLoop

Take the chance to identify and drop modules importing event_loop but
not using it.

Change-Id: Ifa1940cd00138ebc72fbcfd45a0e28246f7a5ed9
diff --git a/src/osmo_gsm_tester/powersupply_sispm.py b/src/osmo_gsm_tester/powersupply_sispm.py
index 91604b4..b35592f 100644
--- a/src/osmo_gsm_tester/powersupply_sispm.py
+++ b/src/osmo_gsm_tester/powersupply_sispm.py
@@ -20,7 +20,8 @@
 import sispm
 from usb.core import USBError
 
-from . import log, event_loop
+from . import log
+from .event_loop import MainLoop
 from .powersupply import PowerSupply
 
 class PowerSupplySispm(PowerSupply):
@@ -50,7 +51,7 @@
             except USBError as e:
                     if e.errno == 16 or e.errno==110:
                         self.log('skip usb error, retry', repr(e))
-                        event_loop.sleep(self, 0.1)
+                        MainLoop.sleep(self, 0.1)
                         continue
                     raise e