runtime: delete redundant code in the page allocator
The page allocator's scavenge index has sysGrow called on it twice,
once in pageAlloc.grow, and once in pageAlloc.sysGrow on 64-bit
platforms. Calling it twice is OK since sysGrow is idempotent,
but it's also wasteful. This change removes the call
in pageAlloc.sysGrow.
Change-Id: I5b955b6e2beed5c2b8305ab82b76718ea305792c
Reviewed-on: https://go-review.googlesource.com/c/go/+/707735
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>