idct.h: remove dead declarations
idct.h: remove dead declarations

Change-Id: Iac7c99eee00a426d115af5343c776cd1f2c5be70

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom