Remove unused CSS (similar to BS4's .align-items-center)

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

app/assets/stylesheets/framework/common.scss

@@ -407,12 +407,6 @@ img.emoji {
 .ws-normal { white-space: normal; }
 .overflow-auto { overflow: auto; }
 
-.d-flex-center {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-}
-
 /** COMMON SIZING CLASSES **/
 .w-0 { width: 0; }