[dev.link] cmd/link: drop hash maps after loading

[dev.link] cmd/link: drop hash maps after loading

The hash maps are used to deduplicate hashed symbols. Once we
loaded all the symbols, we no longer need the hash maps. Drop
them.

Linking cmd/compile,

name         old live-B     new live-B     delta
Loadlib_GC      13.1M ± 0%     11.3M ± 0%   -13.62%  (p=0.008 n=5+5)

Change-Id: I4bb1f84e1111a56d9e777cd6a68f7d974b60e321
Reviewed-on: https://go-review.googlesource.com/c/go/+/245721
Reviewed-by: Jeremy Faller <jeremy@golang.org>