Update comment above BasicStringPiece

A comment above BasicStringPiece definition currently suggests
against using that class directly, confusingly mentioning
BasicStringPiece again as one of the alternatives.

This confusion comes from http://crrev.com/187793004: before
that change, the comment was attached to StringPieceDetail,
thus advising to replace the use of StringPieceDetail with
(among others) BasicStringPiece. However, that CL replaced
StringPieceDetail with BasicStringPiece, making the comment
obsolete.

This CL just deletes the obsolete sentence.

Change-Id: I516e2a14ee846ecf2644097f2ecbc201d2b6f022
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1908267
Auto-Submit: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714240}

base/strings/string_piece.h

@@ -154,8 +154,7 @@ BASE_EXPORT void AssertIteratorsInOrder(string16::const_iterator begin,
 // BasicStringPiece ------------------------------------------------------------
 
 // Defines the types, methods, operators, and data members common to both
-// StringPiece and StringPiece16. Do not refer to this class directly, but
-// rather to BasicStringPiece, StringPiece, or StringPiece16.
+// StringPiece and StringPiece16.
 //
 // This is templatized by string class type rather than character type, so
 // BasicStringPiece<std::string> or BasicStringPiece<base::string16>.