Remove the init_done local variable.

We only call initialize_dec() and av1_initialize_enc() via once(), so
the init_done local variable in initialize_dec() and
av1_initialize_enc() is redundant.

Change-Id: I35e9e27291fa01aff1358536861c432b0717e856
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index 86101b1..36885c1 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -707,6 +707,7 @@
   int default_interp_skip_flags;
 } AV1_COMP;
 
+// Must not be called more than once.
 void av1_initialize_enc(void);
 
 struct AV1_COMP *av1_create_compressor(AV1EncoderConfig *oxcf,