Map delete must clear value every time because
newly added map optimizations of compound-assignment
operators (CL #91557) rely on this behavior of map delete.

It slows down map delete operation for non-reference types:

name                   old time/op    new time/op    delta
MapDelete/Int32/100      23.9ns ± 2%    27.8ns ± 4%  +16.04%  (p=0.000 n=20+20)
MapDelete/Int32/1000     21.5ns ± 2%    25.2ns ± 2%  +17.06%  (p=0.000 n=20+19)
MapDelete/Int32/10000    24.2ns ± 6%    27.2ns ± 5%  +12.39%  (p=0.000 n=19+19)
MapDelete/Int64/100      24.2ns ± 4%    27.7ns ± 2%  +14.55%  (p=0.000 n=20+20)
MapDelete/Int64/1000     22.1ns ± 2%    24.8ns ± 2%  +12.36%  (p=0.000 n=10+20)

Fixes #25936

Change-Id: I8499b790cb5bb019938161b3e50f3243d9bbb79c
Reviewed-on: https://go-review.googlesource.com/120255
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>