runtime: avoid zeroing hmap fields in makemap twice

runtime: avoid zeroing hmap fields in makemap twice

Stack allocated hmap structs are explicitly zeroed before being
passed by pointer to makemap.

Heap allocated hmap structs are created with newobject
which also zeroes on allocation.

Therefore, setting the hmap fields to 0 or nil is redundant
since they will have been zeroed when hmap was allocated.

Change-Id: I5fc55b75e9dc5ba69f5e3588d6c746f53b45ba66
Reviewed-on: https://go-review.googlesource.com/56291
Reviewed-by: Keith Randall <khr@golang.org>