fix(material/datepicker): use direction-agnostic text-align

Avoids some extra styles by using `text-align: start` instead of having to flip it manually for RTL.

(cherry picked from commit 27df9cf)

src/material/datepicker/calendar-body.scss

@@ -57,7 +57,7 @@ $_tokens: (tokens-mat-datepicker.$prefix, tokens-mat-datepicker.get-token-slots(
 .mat-calendar-body-label {
   height: 0;
   line-height: 0;
-  text-align: left;
+  text-align: start;
   padding-left: $calendar-body-label-side-padding;
   padding-right: $calendar-body-label-side-padding;
 
@@ -424,9 +424,3 @@ $_tokens: (tokens-mat-datepicker.$prefix, tokens-mat-datepicker.get-token-slots(
     }
   }
 }
-
-[dir='rtl'] {
-  .mat-calendar-body-label {
-    text-align: right;
-  }
-}