emitter: Remove unimplemented prototype

src/common/x64/emitter.h

@@ -175,7 +175,6 @@ struct OpArg
     void WriteRex(XEmitter *emit, int opBits, int bits, int customOp = -1) const;
     void WriteVex(XEmitter* emit, X64Reg regOp1, X64Reg regOp2, int L, int pp, int mmmmm, int W = 0) const;
     void WriteRest(XEmitter *emit, int extraBytes=0, X64Reg operandReg=INVALID_REG, bool warn_64bit_offset = true) const;
-    void WriteFloatModRM(XEmitter *emit, FloatOp op);
     void WriteSingleByteOp(XEmitter *emit, u8 op, X64Reg operandReg, int bits);
     // This one is public - must be written to
     u64 offset;  // use RIP-relative as much as possible - 64-bit immediates are not available.