electrum: cleanup plugin imports

`electrum_plugins` is imported in the `electrum` script
without being used - I assume it's done to make PyInstaller
find that package. But this can be achieved by avoiding
the usage of `__import__()` when importing `electrum_plugins`
where it's effectively used.

lib/plugins.py

@@ -50,7 +50,7 @@ class Plugins(DaemonThread):
             find = imp.find_module('plugins')
             plugins = imp.load_module('electrum_plugins', *find)
         else:
-            plugins = __import__('electrum_plugins')
+            import electrum_plugins as plugins
         self.pkgpath = os.path.dirname(plugins.__file__)
         self.config = config
         self.hw_wallets = {}