Remove all blank "Modified by:" lines from top comment blocks

Having this line is not useful at all as it doesn't contain any
information and shouldn't be filled in the future as git-shortlog can
provide the information about people who changed the given file more
more reliably than consulting the comments in any case.

Keep the non-blank lines for historical purposes.

samples/html/about/about.cpp

@@ -2,7 +2,6 @@
 // Name:        about.cpp
 // Purpose:     wxHtml sample: about dialog test
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence

samples/html/help/help.cpp

@@ -2,7 +2,6 @@
 // Name:        help.cpp
 // Purpose:     wxHtml sample: help test
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence

samples/html/helpview/helpview.cpp

@@ -2,7 +2,6 @@
 // Name:        helpview.cpp
 // Purpose:     wxHtml sample: help browser
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence

samples/html/virtual/virtual.cpp

@@ -2,7 +2,6 @@
 // Name:        virtual.cpp
 // Purpose:     wxHtml sample: demonstrates virtual file systems feature
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence

samples/html/widget/widget.cpp

@@ -2,7 +2,6 @@
 // Name:        zip.cpp
 // Purpose:     wxHtml sample: Demonstrates embedded controls
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence

samples/html/zip/zip.cpp

@@ -2,7 +2,6 @@
 // Name:        zip.cpp
 // Purpose:     wxHtml sample
 // Author:      ?
-// Modified by:
 // Created:     ?
 // Copyright:   (c) wxWidgets team
 // Licence:     wxWindows licence