Since MR !554 (#15454) GHCi automatically enabled the flag `-fobject-code` on
any module using the UnboxedTuples or UnboxedSum extensions.

MR !1553 (#16876) allowed to inhibit the automatic compiling to object-code
of these modules by setting the `fbyte-code` flag. However, it assigned 2
different semantics to this flag and introduced the regression described in
issue #18955.

This MR fixes this regression by unsetting the internal flag
`Opt_ByteCodeIfUnboxed` before it's copied to DynFlags local to the module.
In GHC 9.0.1 the issue is solved by introducing a new flag
`-f(no-)object-code-if-unboxed`.