Do not remove WAYLAND_DISPLAY environment variable for input method.

Based on the implementation of wl_display_connect, WAYLAND_SOCKET is
always preferred over WAYLAND_DISPLAY, which means it is OK to have both
of them set. This allows subprocess of input method to have the correct
WAYLAND_DISPLAY variable set.

src/inputmethod.cpp

@@ -688,7 +688,6 @@ void InputMethod::startInputMethod()
     QProcessEnvironment environment = kwinApp()->processStartupEnvironment();
     environment.insert(QStringLiteral("WAYLAND_SOCKET"), QByteArray::number(socket));
     environment.insert(QStringLiteral("QT_QPA_PLATFORM"), QStringLiteral("wayland"));
-    environment.remove("WAYLAND_DISPLAY");
 
     m_inputMethodProcess = new QProcess(this);
     m_inputMethodProcess->setProcessChannelMode(QProcess::ForwardedErrorChannel);