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/file/filetest.cpp

@@ -141,11 +141,6 @@ void FileTestCase::TempFile()
 // Check that GetSize() works correctly for special files.
 TEST_CASE("wxFile::Special", "[file][linux][special-file]")
 {
-    // LXC containers don't (always) populate /proc and /sys, so skip these
-    // tests there.
-    if ( IsRunningInLXC() )
-        return;
-
     // We can't test /proc/kcore here, unlike in the similar
     // wxFileName::GetSize() test, as wxFile must be able to open it (at least
     // for reading) and usually we don't have the permissions to do it.