Make «ansible-vault view» not write plaintext to a tempfile

CLI already provides a pager() method that feeds $PAGER on stdin, so we
just feed that the plaintext from the vault file. We can also eliminate
the redundant and now-unused shell_pager_command method in VaultEditor.

lib/ansible/cli/vault.py

@@ -138,7 +138,7 @@ class VaultCLI(CLI):
     def execute_view(self):
 
         for f in self.args:
-            self.editor.view_file(f)
+            self.pager(self.editor.plaintext(f))
 
     def execute_rekey(self):
         for f in self.args: