Remove checks for LXC containers used under Travis CI only

As we don't use Travis CI any longer, we don't need these checks
neither.

tests/test.cpp

@@ -424,20 +424,6 @@ extern bool IsRunningUnderXVFB()
     return s_isRunningUnderXVFB == 1;
 }
 
-#ifdef __LINUX__
-
-extern bool IsRunningInLXC()
-{
-    // We're supposed to be able to detect running in LXC by checking for
-    // /dev/lxd existency, but this doesn't work under Travis for some reason,
-    // so just rely on having the environment variable defined for the
-    // corresponding builds in our .travis.yml.
-    wxString value;
-    return wxGetEnv("wxLXC", &value) && value == "1";
-}
-
-#endif // __LINUX__
-
 #if wxUSE_GUI
 
 bool EnableUITests()