remove option to attach modal dialogs

This was added only to add support for metacity theme format version 3.2.

Adapted from metacity for marco
origin commit: https://gitlab.gnome.org/GNOME/metacity/commit/d2b13461

Author: vkareh

src/core/prefs.c

@@ -335,12 +335,6 @@ static MetaEnumPreference preferences_enum[] =
 
 static MetaBoolPreference preferences_bool[] =
   {
-    { "attach-modal-dialogs",
-      KEY_GENERAL_SCHEMA,
-      META_PREF_ATTACH_MODAL_DIALOGS,
-      &attach_modal_dialogs,
-      TRUE,
-    },
     { "raise-on-click",
       KEY_GENERAL_SCHEMA,
       META_PREF_RAISE_ON_CLICK,

src/org.mate.marco.gschema.xml

@@ -146,11 +146,6 @@
       <summary>Visual Bell Type</summary>
       <description>Tells Marco how to implement the visual indication that the system bell or another application 'bell' indicator has been rung.  Currently there are two valid values, "fullscreen", which causes a fullscreen white-black flash, and "frame_flash" which causes the titlebar of the application which sent the bell signal to flash.  If the application which sent the bell is unknown (as is usually the case for the default "system beep"), the currently focused window's titlebar is flashed.</description>
     </key>
-    <key name="attach-modal-dialogs" type="b">
-      <default>false</default>
-      <summary>Attach modal dialogs</summary>
-      <description>When true, instead of having independent titlebars, modal dialogs appear attached to the titlebar of the parent window and are moved together with the parent window.</description>
-    </key>
     <key name="compositing-manager" type="b">
       <default>true</default>
       <summary>Compositing Manager</summary>