cmake: remove stale comment on precompiled headers

In commit 9f75a9ce7 (Turning on runtime checks when building debug under
MSVC., 2012-03-30), we introduced a comment "Precompiled headers", which
actually refers to no related commands. Seeing that the comment never
had anything to refer to, we can simply remove it here.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 354ae3e..259bd96 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -468,9 +468,6 @@
 	SET(WIN_RC "src/win32/git2.rc")
-   # Precompiled headers
 ELSE ()