Fix `use_fixed_qp_offsets` comment

The `fixed_qp_offsets` array has been deprecated, so its values
aren't actually applied to frame QPs.

Change-Id: Ie9d556a7cb5c6dc94b3f5fbc5e89620ed66edc57

av1/encoder/encoder.h

diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index a5b334f..28c07c0 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -793,9 +793,7 @@
 typedef struct {
   // Controls how the encoder applies fixed QP offsets.
   // If the value is 0, QP offsets are chosen adaptively.
-  // If the value is 1, fixed QP offsets are either:
-  // - Given by the user, and stored in 'fixed_qp_offsets' array, OR
-  // - Picked automatically from cq_level.
+  // If the value is 1, fixed QP offsets are picked automatically from cq_level.
   // If the value is 2, no QP offsets will be applied.
   int use_fixed_qp_offsets;
   // Indicates the minimum flatness of the quantization matrix.