Revert "Use Qt's own glyph cache with the freetype engine."

This reverts commit 8c58640fdf.

The intent was to reduce the maintenance of parallel code paths,
but this had a negative impact on performance so we need to
enable it again.

Change-Id: Ic58f7781ff076055e7c48f20fc0e7a8a2c9e51a4
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>

src/gui/text/qfontengine_ft.cpp

@@ -631,7 +631,8 @@ QFontEngineFT::QFontEngineFT(const QFontDef &fd)
 #endif
     defaultFormat = Format_None;
     embeddedbitmap = false;
-    cacheEnabled = qEnvironmentVariableIsSet("QT_USE_FT_CACHE");
+    const QByteArray env = qgetenv("QT_NO_FT_CACHE");
+    cacheEnabled = env.isEmpty() || env.toInt() == 0;
     m_subPixelPositionCount = 4;
 }