After the --wasm-shared-engine flag was removed (in
https://crrev.com/c/1864935), there is no point any more in holding the
wasm engine in a shared_ptr. The engine is initialized once for the
whole process, and only deallocated during global tear down.

R=jkummerow@chromium.org

Bug: v8:11384
Change-Id: Id8e96eaecfcab8b44842ec323c94529e9c5a5e25
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2853589Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74253}