[wasm][interpreter] Remove code for calling imports

Since the interpreter is only used for internal testing now, it does not
have an associated frame type any more. We thus cannot call external
functions any more, since a stack walk would not know how to handle
interpreter frames.

All code for calling external (imported) functions is dead by now, so
this CL removes it.

R=thibaudm@chromium.org

Bug: v8:10389
Change-Id: If4b56b93e63002a1e5463f278f0271eb413762eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2218287
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68039}