Remove design variable `footer-height`

app/assets/stylesheets/layout/_base_mobile.sass

@@ -40,7 +40,7 @@
     padding-bottom: 0 !important
     position: relative !important
     top: 0 !important
-    min-height: calc(100vH - #{$header-height-mobile} - #{$footer-height})
+    min-height: calc(100vH - #{$header-height-mobile})
 
   #main
     height: initial !important

app/assets/stylesheets/layout/_work_package_table.sass

@@ -104,7 +104,7 @@
 
 // Footer of the left side
 .work-packages-split-view--tabletimeline-footer
-  flex: 0 0 $footer-height
+  flex: 0 0 55px
 
   // Only add padding to lower/right corner
   // to override default margins

lib/open_project/design.rb

@@ -77,7 +77,6 @@ module OpenProject
       'header-search-field-bg-color'                         => "#FFFFFF",
       'header-search-field-font-color'                       => "#000000",
       'header-search-field-border'                           => "0",
-      'footer-height'                                        => "55px",
       'main-menu-width'                                      => "230px",
       'main-menu-folded-width'                               => "50px",
       'main-menu-border-color'                               => "#E7E7E7",