content_encoding: drop a guard for brotli 1.0.0+ macro

Also add comment with version requirement for the other guard.

Refs:
https://github.com/google/brotli/commit/19d86fb9a60aa7034d4981b69a5b656f5b90017e
https://github.com/google/brotli/commit/03739d2b113afe60638069c4e1604dc2ac27380d

Ref: #19672
Follow-up to e639d4ca4d794c222dde4680d9ff35053f501042
Closes #19673
diff --git a/lib/content_encoding.c b/lib/content_encoding.c
index b724b57..f128f75 100644
--- a/lib/content_encoding.c
+++ b/lib/content_encoding.c
@@ -384,12 +384,10 @@
   case BROTLI_DECODER_ERROR_FORMAT_WINDOW_BITS:
   case BROTLI_DECODER_ERROR_FORMAT_PADDING_1:
   case BROTLI_DECODER_ERROR_FORMAT_PADDING_2:
-#ifdef BROTLI_DECODER_ERROR_COMPOUND_DICTIONARY
+#ifdef BROTLI_DECODER_ERROR_COMPOUND_DICTIONARY  /* brotli v1.1.0+ */
   case BROTLI_DECODER_ERROR_COMPOUND_DICTIONARY:
 #endif
-#ifdef BROTLI_DECODER_ERROR_DICTIONARY_NOT_SET
   case BROTLI_DECODER_ERROR_DICTIONARY_NOT_SET:
-#endif
   case BROTLI_DECODER_ERROR_INVALID_ARGUMENTS:
     return CURLE_BAD_CONTENT_ENCODING;
   case BROTLI_DECODER_ERROR_ALLOC_CONTEXT_MODES: