Hash: Remove the HashFNV prototype

It has no function definition.

Source/Core/Common/Hash.h

@@ -10,7 +10,6 @@
 
 u32 HashFletcher(const u8* data_u8, size_t length);  // FAST. Length & 1 == 0.
 u32 HashAdler32(const u8* data, size_t len);         // Fairly accurate, slightly slower
-u32 HashFNV(const u8* ptr, int length);              // Another fast and decent hash
 u32 HashEctor(const u8* ptr, int length);            // JUNK. DO NOT USE FOR NEW THINGS
 u64 GetCRC32(const u8 *src, u32 len, u32 samples);   // SSE4.2 version of CRC32
 u64 GetHashHiresTexture(const u8 *src, u32 len, u32 samples = 0);