Revert "build: for default bootstrap, use Go 1.17 if present, falling back to Go...

Revert "build: for default bootstrap, use Go 1.17 if present, falling back to Go 1.4"

This reverts https://golang.org/cl/369914.

Reason for revert: Breaking previously working toolchain builds.

For #44505.

Change-Id: I09ae20e50109a600d036358118077d27669df39c
Reviewed-on: https://go-review.googlesource.com/c/go/+/370138
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>