Eliminate internal use of GETJOCTET() macro

Because of 01e3032354 (officially
eliminating support for compilers without unsigned char, since we never
effectively supported those compilers anyhow), GETJOCTET() is now a
no-op.  Since that macro is in jmorecfg.h, it is part of the de facto
libjpeg API and must remain in the public headers.  However, there is no
reason to continue using it internally, and eliminating its internal use
improves code readability.

djpeg.c

@@ -444,7 +444,7 @@ jpeg_getc(j_decompress_ptr cinfo)
       ERREXIT(cinfo, JERR_CANT_SUSPEND);
   }
   datasrc->bytes_in_buffer--;
-  return GETJOCTET(*datasrc->next_input_byte++);
+  return *datasrc->next_input_byte++;
 }