With the recent removal of the --wasm-shared-code flag, it became
effectively impossible to turn off this flag. Hence its functionality
became mandatory and the ability to turn off sharing of {WasmEngine}
process-wide has to be removed as well.

R=clemensb@chromium.org

Change-Id: I7c25e909e49134a226d6a9fe9c42f0ecd9d02a69
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864935
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64322}