common: fix a comment

[mrFranklin]

this method is in use, not Deprecated.

[MariusVanDerWijden]

Well the point of this comment is, that the method should not be in use

[mrFranklin]

@MariusVanDerWijden @holiman
But the method can not be deleletd. It's in use.
Should we delete it?

[mrFranklin]

the method is not the same as hexutil.Encode. so I think it's useful.
as it say:
For empty slices, the return value is "0x0".

hexutil.Encode can not.

[fjl]

I marked ToHex deprecated a long time ago when I tried to remove as much hex encoding/decoding as possible from package common. The replacement for these functions is/was package hexutil.

The 0x0 special case for []byte is very weird and I hope we are not using it anyhwere. The only thing that should encode to 0x0 is the integer zero.