MDL-68253 mod_book: Remove prev arrow on the firstmost page.

mod/book/lib.php

@@ -725,11 +725,9 @@ function mod_book_get_fontawesome_icon_map() {
     return [
         'mod_book:chapter' => 'fa-bookmark-o',
         'mod_book:nav_prev' => 'fa-arrow-left',
-        'mod_book:nav_prev_dis' => 'fa-angle-left',
         'mod_book:nav_sep' => 'fa-minus',
         'mod_book:add' => 'fa-plus',
         'mod_book:nav_next' => 'fa-arrow-right',
-        'mod_book:nav_next_dis' => 'fa-angle-right',
         'mod_book:nav_exit' => 'fa-arrow-up',
     ];
 }

mod/book/view.php

@@ -152,7 +152,6 @@ foreach ($chapters as $ch) {
 if ($book->navstyle) {
     $navprevicon = right_to_left() ? 'nav_next' : 'nav_prev';
     $navnexticon = right_to_left() ? 'nav_prev' : 'nav_next';
-    $navprevdisicon = right_to_left() ? 'nav_next_dis' : 'nav_prev_dis';
 
     $chnavigation = '';
     if ($previd) {
@@ -167,10 +166,6 @@ if ($book->navstyle) {
                 '<span class="chaptername"><span class="arrow">' . $OUTPUT->larrow() . '&nbsp;</span></span>' .
                 $navprev . ':&nbsp;<span class="chaptername">' . $prevtitle . '</span></a>';
         }
-    } else {
-        if ($book->navstyle == 1) {
-            $chnavigation .= $OUTPUT->pix_icon($navprevdisicon, '', 'mod_book');
-        }
     }
     if ($nextid) {
         $navnext = get_string('navnext', 'book');