compiler.cc: Remove a comment which is no longer true.

We don't do any automatic PreParsing in Compiler::Compile any more (it was
removed by r10066, > 2 years ago).

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/225993002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20526 ce2b1a6d-e550-0410-aec6-3dcde31c8c00