compiler: remove unused Type::in_heap_ member variable

This member variable was added in https://golang.org/cl/46490, but it
was never used. The code uses Named_type::in_heap_ instead.

Change-Id: I155ddbc35270295a2b66f3b82ea84cdf8a324adb
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/265257
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>