debugger: get rid of the private header dependency

Removed the CanvasFrameRate class as it is not used, and
depends on private Qt API

src/plugins/debugger/qml/qml.pri

@@ -2,13 +2,3 @@
 HEADERS += $$PWD/qmlengine.h
 SOURCES += $$PWD/qmlengine.cpp
 
-include(../../../private_headers.pri)
-exists($${QT_PRIVATE_HEADERS}/QtDeclarative/private/qdeclarativecontext_p.h) {
-    HEADERS += \
-        $$PWD/canvasframerate.h \
-
-    SOURCES += \
-        $$PWD/canvasframerate.cpp \
-
-    QT += declarative
-}