Noticed during an audit, we've got duplicated code that's unused.
There were 2 type definitions:
* type kvPair types.Pair
* type KVPair types.Pair
and each had a .bytes() and .Bytes() method respectively that
then used some extra code.
This change deletes the duplicated/unnecessary code but also
while here improves the performance by removing the use of a
bytes.Buffer which is unnecessary given that we are only
length prefixed the key, length prefixing the value hence
the various helpers are unnecessary.
The added benchmarks shows the performance boost
```shell
$ benchstat before after
name old time/op new time/op delta
KVPairBytes-8 146µs ± 1% 142µs ± 2% -3.05% (p=0.000 n=18+17)
name old speed new speed delta
KVPairBytes-8 6.84GB/s ± 1% 7.06GB/s ± 2% +3.15% (p=0.000 n=18+17)
name old alloc/op new alloc/op delta
KVPairBytes-8 1.01MB ± 0% 1.01MB ± 0% -0.04% (p=0.000 n=17+20)
name old allocs/op new allocs/op delta
KVPairBytes-8 6.00 ± 0% 1.00 ± 0% -83.33% (p=0.000 n=20+20)
```
Closes: #6688
Emmanuel T Odeke
152ac24931