Remove restricted mode (-Z) from man page.

As per #11996, restricted mode (-Z) has been removed. Nonetheless in #11996 the man page was not updated. Hence this PR.

man/nvim.1

@@ -113,9 +113,6 @@ associated with a file.
 To overwrite a file, add an exclamation mark to the relevant Ex command, such as
 .Ic :w! .
 .Ic ":help 'readonly'"
-.It Fl Z
-Restricted mode.
-Disable commands that make use of an external shell.
 .It Fl m
 Resets the 'write' option, to disable file modifications.
 Writing to a file is disabled, but buffers can still be modified.