fix: Remove non-existing bookmarks.{export,import}

These APIs are not supported by Firefox:
https://searchfox.org/mozilla-central/rev/292d295d6b084b43b70de26a42e68513bb7b36a3/browser/components/extensions/schemas/bookmarks.json#436-465

In Chrome the APIs are not available either, except to some internal components:
b7963dca6a/chrome/common/extensions/api/_api_features.json (138)

Also moved "getTree" to ensure that the keys are alphabetically sorted.

api-metadata.json

@@ -22,10 +22,6 @@
       "minArgs": 1,
       "maxArgs": 1
     },
-    "export": {
-      "minArgs": 0,
-      "maxArgs": 0
-    },
     "get": {
       "minArgs": 1,
       "maxArgs": 1
@@ -38,15 +34,11 @@
       "minArgs": 1,
       "maxArgs": 1
     },
-    "getTree": {
-      "minArgs": 0,
-      "maxArgs": 0
-    },
     "getSubTree": {
       "minArgs": 1,
       "maxArgs": 1
     },
-    "import": {
+    "getTree": {
       "minArgs": 0,
       "maxArgs": 0
     },