The relocation mode in question was by now only used in tests to model a
wrapper call from wrapper code (on the GC'ed heap) to a non-movable wasm
code object. Instead of using a special relocation mode, we switch to
using the existing {EXTERNAL_REFERENCE} mode similar to other static C++
functions called from generated code.

R=sigurds@chromium.org
BUG=v8:8238

Change-Id: I30af98b92aed207c52ccccaf018a455ecac39c2b
Reviewed-on: https://chromium-review.googlesource.com/c/1309821Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57239}