Remove unused abbreviate prop

This was accidentally removed in 800b6e0163 but given that we've had that on prod for well over a year we figure it's better to just drop it. Abbreviated dates adds complexity for screen readers as well

Co-Authored-By: Sergio Padrino <1083228+sergiou87@users.noreply.github.com>

app/src/ui/history/commit-list-item.tsx

@@ -227,7 +227,7 @@ function renderRelativeTime(date: Date) {
   return (
     <>
       {` • `}
-      <RelativeTime date={date} abbreviate={true} />
+      <RelativeTime date={date} />
     </>
   )
 }

app/src/ui/relative-time.tsx

@@ -10,14 +10,6 @@ interface IRelativeTimeProps {
    */
   readonly date: Date
 
-  /**
-   * For relative durations, use abbreviated units
-   * ('m' instead of 'minutes', 'd' instead of 'days')
-   *
-   * Defaults to `false`
-   */
-  readonly abbreviate?: boolean
-
   /**
    * By default the RelativeTime component will start displaying a compact
    * absolute date if the date is more than one week ago. Setting `onlyRelative`