Remove workaround for now-fixed Clang CPP bug (#1028)

Before LLVM 6.0.1 (or 10.0 on Apple LLVM), there was a bug where
lines that started with an octothorpe but turned out not
to lex like pragmas would have an extra line added after them.

Since this bug has been fixed upstream and that it doesn't have dire
consequences anyways, the workaround is not really worth it
anymore - we can just tell people to update their clang version (or re-structure
their pragma code).