Revert "fmt: fix incorrect format of whole-number floats when using %#v"

Numbers without decimals are valid Go representations of whole-number
floats. That is, "var x float64 = 5" is valid Go. Avoid breakage in
tests that expect a certain output from %#v by reverting to it.

To guarantee the right type is generated by a print use %T(%#v) instead.

Added a test to lock in this behavior.

This reverts commit 7c7cecc1.

Fixes #27634
Updates #26363

Change-Id: I544c400a0903777dd216452a7e86dfe60b0b0283
Reviewed-on: https://go-review.googlesource.com/c/142597


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>