MDL-73700 core: remove unused functions and strings from installer

While searching for no longer needed PHP 5.x & 7.0 checks, the
`phpversionhelp` lang string and, subsequently, the `memorylimithelp`
lang string and the `install_helpbutton()` and
`install_print_help_page()` functions were discovered. According to
comments on MDL-2787 and MDL-64482 they are no longer in use since
Moodle 1.x (approximately), neither reachable since some good versions
ago. With MDL-17458 (commit 3b09331066),
the function `print_compatibility_row()` was gone, and it seems like it
was the only place calling `install_helpbutton()`. And
`install_helpbutton()` was the only place where a link to the help mode
(e.g.  `install.php?help=$helpfield`) ever surfaced.  As both lang
strings and functions are particular to the installer and never could
have been used outside of it, we are going for direct deletion instead
of regular deprecation.

Signed-off-by: Daniel Ziegenberg <[email protected]>

install.php

@@ -240,11 +240,6 @@ $hint_dataroot = '';
 $hint_admindir = '';
 $hint_database = '';
 
-// Are we in help mode?
-if (isset($_GET['help'])) {
-    install_print_help_page($_GET['help']);
-}
-
 //first time here? find out suitable dataroot
 if (is_null($CFG->dataroot)) {
     $CFG->dataroot = __DIR__.'/../moodledata';