Doc: Remove mentioning of qstringbuilder.cpp

The file is actually in src/corelib/text, and the majority
of the implementation is in the header file.

In any case, I don't think mentioning the name here is good
practice. Those who are a bit into Qt probably can find the
implementation quite easily themselves.

Change-Id: I7c7f0d655b55b509a830e92a02940f7b91df2dea
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: hjk <hjk@qt.io>

src/corelib/text/qstring.cpp

@@ -2066,9 +2066,7 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
     \c{QStringBuilder}. This class is marked
     internal and does not appear in the documentation, because you
     aren't meant to instantiate it in your code. Its use will be
-    automatic, as described below. The class is found in
-    \c {src/corelib/tools/qstringbuilder.cpp} if you want to have a
-    look at it.
+    automatic, as described below.
 
     \c{QStringBuilder} uses expression templates and reimplements the
     \c{'%'} operator so that when you use \c{'%'} for string