Fix various Clang 7 warnings about unused stuff

Namely:
[-Wunneeded-internal-declaration]
[-Wunused-comparison]
[-Wunused-const-variable]
[-Wunused-function]
[-Wunused-private-fields]

Fixes the following Clang 7 warnings:
```
editor/plugins/script_editor_plugin.cpp:1417:20: warning: function '_find_node_with_script' is not needed and will not be emitted [-Wunneeded-internal-declaration]
editor/scene_tree_dock.cpp:1859:14: warning: function '_find_last_visible' is not needed and will not be emitted [-Wunneeded-internal-declaration]

modules/gdscript/gdscript_parser.cpp:7838:19: warning: equality comparison result unused [-Wunused-comparison]

scene/resources/mesh.cpp:549:35: warning: unused variable '_array_types' [-Wunused-const-variable]
scene/resources/mesh.cpp:563:18: warning: unused variable '_format_translate' [-Wunused-const-variable]

drivers/gles3/rasterizer_scene_gles3.cpp:54:28: warning: unused function 'store_transform2d' [-Wunused-function]

core/io/file_access_network.h:50:6: warning: private field 'ml' is not used [-Wunused-private-field]
core/io/file_access_zip.h:93:14: warning: private field 'archive' is not used [-Wunused-private-field]
core/io/resource_format_binary.h:122:6: warning: private field 'bin_meta_idx' is not used [-Wunused-private-field]
core/message_queue.h:47:9: warning: private field 'mutex' is not used [-Wunused-private-field]
main/tests/test_gui.cpp:63:11: warning: private field 'control' is not used [-Wunused-private-field]
modules/gdscript/gdscript_parser.h:558:7: warning: private field 'completion_static' is not used [-Wunused-private-field]
platform/x11/os_x11.h:148:11: warning: private field 'ip_unix' is not used [-Wunused-private-field]
platform/x11/os_x11.h:180:7: warning: private field 'net_wm_icon' is not used [-Wunused-private-field]
platform/x11/os_x11.h:189:6: warning: private field 'audio_driver_index' is not used [-Wunused-private-field]
platform/x11/os_x11.h:190:15: warning: private field 'capture_idle' is not used [-Wunused-private-field]
servers/physics/body_pair_sw.h:79:6: warning: private field 'cc' is not used [-Wunused-private-field]
servers/visual/visual_server_raster.h:62:7: warning: private field 'draw_extra_frame' is not used [-Wunused-private-field]
```

3 files changed, 0 insertions, 5 deletions

diff --git a/core/io/file_access_network.h b/core/io/file_access_network.h
index 7e4669ffd5..e32dcea990 100644
--- a/core/io/file_access_network.h
+++ b/core/io/file_access_network.h
@@ -47,8 +47,6 @@ class FileAccessNetworkClient {
 		int size;
 	};
 
-	int ml;
-
 	List<BlockRequest> block_requests;
 
 	Semaphore *sem;
diff --git a/core/io/file_access_zip.h b/core/io/file_access_zip.h
index 4fe0651a55..9bb1ad221a 100644
--- a/core/io/file_access_zip.h
+++ b/core/io/file_access_zip.h
@@ -90,8 +90,6 @@ class FileAccessZip : public FileAccess {
 
 	mutable bool at_eof;
 
-	ZipArchive *archive;
-
 public:
 	virtual Error _open(const String &p_path, int p_mode_flags); ///< open a file
 	virtual void close(); ///< close a file
diff --git a/core/io/resource_format_binary.h b/core/io/resource_format_binary.h
index 35d594f228..513252055f 100644
--- a/core/io/resource_format_binary.h
+++ b/core/io/resource_format_binary.h
@@ -119,7 +119,6 @@ class ResourceFormatSaverBinaryInstance {
 	bool skip_editor;
 	bool big_endian;
 	bool takeover_paths;
-	int bin_meta_idx;
 	FileAccess *f;
 	String magic;
 	Set<RES> resource_set;