Fix LOG_EVERY_N with clang -Wunused-local-typedef

Fix LOG_EVERY_N with clang -Wunused-local-typedef

Glog uses a pre-C++11 compile time assert to verify the validity of
the severity parameter for LOG_EVERY_N. Unfortunately, some compilers
will complain about the usage of LOG_EVERY_N with
"-Wunused-local-typedef" due to the way the compile time assert is
constructed. This makes it impossible to use LOG_EVERY_N with this
warning treated as an error.

The fix simply removes the assert entirely. This is safe to do since
you can't put anything invalid into the severity parameters without
generating a compile error elsewhere. This has been safe to do ever
since the GLOG_ prefixes were added as part of 6febec361e.

Fixes #223