sessionMode: Drop allowExtensions property

Now that we allow extensions at the lock screens, extensions
are allowed for every session mode gnome-shell would typically
change to at runtime.

This means there's little advantage to having an allowExtensions
property in the session mode definition.

This commit simplifies the code a bit by dropping the property.

Third party session modes can still lock down extensions through
gsettings if they need to.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1967>

js/ui/sessionMode.js

@@ -20,7 +20,6 @@ const _modes = {
         showCalendarEvents: false,
         showWelcomeDialog: false,
         allowSettings: false,
-        allowExtensions: false,
         allowScreencast: false,
         enabledExtensions: [],
         hasRunDialog: false,
@@ -42,7 +41,6 @@ const _modes = {
     },
 
     'gdm': {
-        allowExtensions: true,
         hasNotifications: true,
         isGreeter: true,
         isPrimary: true,
@@ -59,7 +57,6 @@ const _modes = {
     },
 
     'unlock-dialog': {
-        allowExtensions: true,
         isLocked: true,
         unlockDialog: undefined,
         components: ['polkitAgent', 'telepathyClient'],
@@ -76,7 +73,6 @@ const _modes = {
         showCalendarEvents: true,
         showWelcomeDialog: true,
         allowSettings: true,
-        allowExtensions: true,
         allowScreencast: true,
         hasRunDialog: true,
         hasWorkspaces: true,