Remove unused FlMockTextInputHandler (#56495)

This was the only thing that required FlTextInputHandler to be derivable, so also simplify the class.

Author: robert-ancell

shell/platform/linux/BUILD.gn

@@ -252,7 +252,6 @@ executable("flutter_linux_unittests") {
     "testing/mock_renderer.cc",
     "testing/mock_settings.cc",
     "testing/mock_signal_handler.cc",
-    "testing/mock_text_input_handler.cc",
     "testing/mock_text_input_view_delegate.cc",
     "testing/mock_texture_registrar.cc",
     "testing/mock_window.cc",

shell/platform/linux/fl_keyboard_manager_test.cc

@@ -16,7 +16,6 @@
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_standard_method_codec.h"
 #include "flutter/shell/platform/linux/testing/fl_test.h"
 #include "flutter/shell/platform/linux/testing/mock_keymap.h"
-#include "flutter/shell/platform/linux/testing/mock_text_input_handler.h"
 #include "flutter/testing/testing.h"
 
 #include "gmock/gmock.h"