find_editor: remove the env_error argument

There is no case where we should fail to find an editor just because
the values of EDITOR, VISUAL and SUDO_EDITOR are unavailable.  Both
sudoedit and the "env_editor" sudoers setting are documented as
falling back on the hard-coded list of editors in the "editors"
sudoers setting.  Bug #1000

plugins/sudoers/sudoers.h

@@ -445,7 +445,7 @@ extern const char *path_plugin_dir;
 
 /* editor.c */
 char *find_editor(int nfiles, char **files, int *argc_out, char ***argv_out,
-     char * const *allowlist, const char **env_editor, bool env_error);
+     char * const *allowlist, const char **env_editor);
 
 /* exptilde.c */
 bool expand_tilde(char **path, const char *user);