cmd/compile: remove types.Sym.lsym

cmd/compile: remove types.Sym.lsym

It was just a cache, and the CL series yesterday
removed 40% of the calls to types.Linksym in make.bash.

Testing atop CL 40693 (backend concurrency)
indicates that removing it is actually a very minor
performance improvement.

Passes toolstash-check.

Change-Id: I97c2973036964acdd11b3cb842bc31f33ae60389
Reviewed-on: https://go-review.googlesource.com/41492
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>