[WebAssembly] Remove dominator dependency in WasmEHPrepare (NFC)

Dominator trees were previously used for an optimization related to
`wasm.lsda` but the optimization was removed in D97309. Currently
dominators are not doing anything in this pass. Also removes some
`include` lines without which it compiles.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D106811