[lld] [lld][WebAssembly] Don't set importUndefined when -shared is used. NFC (PR #75241)

[Sam Clegg via llvm-commits]

sbc100 wrote:

`importUndefined` corresponds to `--import-undefined` which is documented as `Turn undefined symbols into imports where possible`. 

This also gets set by `--allow-undefined`.

However in the 2 places where we handle this we already handle `isPIC` (see lld/wasm/Relocations.cpp and lld/wasm/Writer.cpp).



https://github.com/llvm/llvm-project/pull/75241