[clang][doc] Removes obsolete comment.

[Mordante]

Summary

After reading about the documentation improvements on LLVM weekly this
part seems obsolete.

[ChuanqiXu]

Sorry. My bad for forgetting this. Thank you very much!

clang/docs/StandardCPlusPlusModules.rst

 Please see https://github.com/llvm/llvm-project/labels/clang%3Amodules for more issues
 or file a new issue if you don't find an existing one.
 If you're going to create a new issue for standard C++ modules,
 please start the title with ``[C++20] [Modules]`` (or ``[C++2b] [Modules]``, etc)
 and add the label ``clang:modules`` (if you have permissions for that).
 
 For higher level support for proposals, you could visit https://clang.llvm.org/cxx_status.html.
 
-Support for clang-scan-deps
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-The support for clang-scan-deps may be the most urgent problem for modules now.
-Without the support for clang-scan-deps, it's hard to involve build systems.
-This means that users could only play with modules through makefiles or by writing a parser by hand.
-It blocks more uses for modules, which will block more defect reports or requirements.
-
-This is tracked in: https://github.com/llvm/llvm-project/issues/51792.
-
 Ambiguous deduction guide
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Currently, when we call deduction guides in global module fragment,
 we may get incorrect diagnosing message like: `ambiguous deduction`.
 
 So if we're using deduction guide from global module fragment, we probably need to write: