cut the folly::vformat comment in CustomLogFormatter

Summary:
The comment was that using named args is impossible with variable types.

But it is likely possible, by having a composite type wrapping a variant and having formatter specializations for each of the alternatives in the variant. So just remove the impossibility comment.

Reviewed By: Gownta

Differential Revision: D66284151

fbshipit-source-id: 3b5ad12d6ed45bcfa298e2d3ef2abad245385e81

folly/logging/CustomLogFormatter.cpp

@@ -125,8 +125,7 @@ void CustomLogFormatter::parseFormatString(StringPiece input) {
   functionNameCount_ = 0;
   fileNameCount_ = 0;
   fileNameNoExtCount_ = 0;
-  // Replace all format keys to numbers to improve performance and to use
-  // varying value types (which is not possible using folly::vformat()).
+  // Replace all format keys to numbers to improve performance.
   std::string output;
   output.reserve(input.size());
   const char* varNameStart = nullptr;