qrenderdoc: Keep the GUI on the xcb Qt platform with Wayland support built in

Building the Wayland windowing support in is what lets renderdoc capture
native Wayland applications, but it also stops qrenderdoc defaulting to
the xcb Qt platform, and upstream says running the GUI directly on
Wayland is unsupported and likely to crash.

The two are independent: keep defaulting the GUI to xcb either way, so a
build with Wayland capture support still runs its own window on the
well-tested path. Setting QT_QPA_PLATFORM still overrides it, and the
unsupported warning is still shown to anyone who does.
--- a/qrenderdoc/Code/qrenderdoc.cpp
+++ b/qrenderdoc/Code/qrenderdoc.cpp
@@ -200,7 +200,7 @@
   if(IsRunningAsAdmin())
     qInfo() << "Running as administrator";
 
-#if defined(RENDERDOC_PLATFORM_LINUX) && !defined(RENDERDOC_WINDOWING_WAYLAND)
+#if defined(RENDERDOC_PLATFORM_LINUX)
   bool envChanged = false;
   {
     const char *qpa_plat = getenv("QT_QPA_PLATFORM");
@@ -609,7 +609,7 @@
       RENDERDOC_InitialiseReplay(env, coreargs);
     }
 
-#if defined(RENDERDOC_PLATFORM_LINUX) && !defined(RENDERDOC_WINDOWING_WAYLAND)
+#if defined(RENDERDOC_PLATFORM_LINUX)
     if(envChanged)
       unsetenv("QT_QPA_PLATFORM");
 #endif