[Switch Access] Remove unavailable action

Performing the standard action "showContextMenu" on an automation node
currently does nothing. This change removes it as an option so as not to
slow down navigation without adding value.

Bug: None
Change-Id: Ia677d8236840580fc153b5e6799055b87ea66504
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814639
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#698680}

chrome/browser/resources/chromeos/switch_access/switch_access_constants.js

@@ -159,8 +159,6 @@ SAConstants.MenuAction = {
   SELECT: 'select',
   // Open and jump to the Switch Access settings.
   SETTINGS: 'settings',
-  // Show the system context menu for the current element.
-  SHOW_CONTEXT_MENU: chrome.automation.ActionType.SHOW_CONTEXT_MENU,
   // Set the end of a text selection.
   SELECT_END: 'selectEnd',
   // Set the beginning of a text selection.