_content/ref/mod: remove old notice on go adding a toolchain directive

Since Go 1.25 this is no longer the case.
See also https://go.dev/doc/go1.25#go-command.

For golang/go#65857

Author: leonklingele

_content/ref/mod.md

@@ -632,9 +632,6 @@ The `toolchain` directive
 only has an effect when the module is the main module and the default toolchain's
 version is less than the suggested toolchain's version.
 
-For reproducibility, the `go` command writes its own toolchain name in a `toolchain` line any time
-it is updating the `go` version in the `go.mod` file (usually during `go get`).
-
 For details, see “[Go toolchains](/doc/toolchain)”.
 
 ```