Revert double buffering.

[methane]

Go 1.21 will fix race between Context cancel and sql.RawBytes.
See golang/go#60304

It will be backported to Go 1.19.

We had implemented double buffering (#943) to workaround it. It increase memory usage and complexity.

After Go 1.19.10 containing this fix is released, let's remove the double buffering in go-mysql-driver v1.8.0.

P.S. We need to drop Go 1.18 support. It would violate our Go version support policy. But Go 1.21 will be released in August, before we release v1.8.0.

[methane]

It is not backported into Go 1.19 and 1.20.
So we need to wait that we drop Go 1.20 support.

[methane]

#1639 removed Go 1.20 support. Now we can drop the double buffering.