MDL-41198 lib: Allow PHP to generate random seed on script start

All mt_srand instances are removed to allow PHP to do the work.

backup/cc/entities.class.php

@@ -309,7 +309,6 @@ class entities {
             $source = str_split($source, 1);
 
             for ($i = 1; $i <= $length; $i++) {
-                mt_srand((double) microtime() * 1000000);
                 $num = mt_rand(1, count($source));
                 $response .= $source[$num - 1];
             }

lib/moodlelib.php

@@ -7650,7 +7650,6 @@ function random_string ($length=15) {
     $pool .= 'abcdefghijklmnopqrstuvwxyz';
     $pool .= '0123456789';
     $poollen = strlen($pool);
-    mt_srand ((double) microtime() * 1000000);
     $string = '';
     for ($i = 0; $i < $length; $i++) {
         $string .= substr($pool, (mt_rand()%($poollen)), 1);
@@ -7671,7 +7670,6 @@ function complex_random_string($length=null) {
     $pool  = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
     $pool .= '`~!@#%^&*()_+-=[];,./<>?:{} ';
     $poollen = strlen($pool);
-    mt_srand ((double) microtime() * 1000000);
     if ($length===null) {
         $length = floor(rand(24, 32));
     }