cmake: remove stale comment on precompiled headers

In commit 9f75a9ce (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.

CMakeLists.txt

@@ -468,9 +468,6 @@ IF (MSVC)
 	SET(CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL "${CMAKE_EXE_LINKER_FLAGS_MINSIZEREL}")
 
 	SET(WIN_RC "src/win32/git2.rc")
-
-   # Precompiled headers
-
 ELSE ()
 	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE ${CMAKE_C_FLAGS}")