Adding back gem.json files to the Atom and AtomLyIntegration folders

which are to be seen as sub gem roots as a workaround for detecting the
location of the "gem" root for the Atom/AtomLyIntegration GEM_MODULE targets

Removed the logic in the SettingsRegistry.cmake for reading a
"gem_module_roots" key from the gem.json file in order to determine the
root of the Atom and Atom LyIntegration sub gem modules

Gems/AtomLyIntegration/TechnicalArt/DccScriptingInterface/gem.json

@@ -0,0 +1,10 @@
+{
+    "gem_name": "DccScriptingInterface",
+    "display_name": "Atom Dcc Scripting Interface",
+    "summary": "",
+    "canonical_tags": [
+        "Gem"
+    ],
+    "user_tags": [
+    ]
+}