output/alsa: use IsCleared() instead of IsDrained() in DrainInternal()

Really drain all data from the period_buffer in any case.

src/lib/alsa/PeriodBuffer.hxx

@@ -77,14 +77,6 @@ public:
 		return tail >= capacity;
 	}
 
-	/**
-	 * Has some data already been consumed at the buffer's head in
-	 * this period?
-	 */
-	bool WasConsumed() const noexcept {
-		return head > 0;
-	}
-
 	/**
 	 * Has all data for the current period been drained?  If not,
 	 * then there is pending data.  This ignores the partial frame