test: cleanup fixedbugs/notinheap.go

[cuonglm]

After #46731, //go:notinheap is gone, however, there's still test/fixedbugs/notinheap.go which is using it. There's no problem with trybot, because the file is ignored:

go/test/run.go

Line 1975 in 34f0029

"fixedbugs/notinheap.go", // types2 doesn't report errors about conversions that are invalid due to //go:notinheap

We should revise those test cases:

• Writing the equivalent ones, replacing //go:notinheap by cgo.Incomplete.
• Remove the file entirely.

cc @mdempsky @randall77

[cuonglm]

Taking a quick look, not all the cases can be rewrite, for example, there's no thing like this anymore:

//go:notinheap
type t3 byte