[lld][WebAssembly] Remove relocation target verification

[lld][WebAssembly] Remove relocation target verification

We have this extra step in wasm-ld that doesn't exist in other lld
backend which verifies the existing contents of the relocation targets.
This was originally intended as an extra form of double checking and an
aid to compiler developers.   However it has always been somewhat
controversial and there have been suggestions in the past the we simply
remove it.

My motivation for removing it now is that its causing me a headache
when trying to fix an issue with negative addends.  In the case of
negative addends that final result can be wrapped/negative but this
checking code would require significant modification to be able to deal
with that case.  For example with some test cases I'm looking at I'm
seeing error like this:

```
wasm-ld: warning: /usr/local/google/home/sbc/dev/wasm/llvm-build/tools/lld/test/wasm/Output/merge-string.s.tmp.o:(.rodata_relocs): unexpected existing value for R_WASM_MEMORY_ADDR_I32: existing=FFFFFFFA expected=FFFFFFFFFFFFFFFA
```

Rather than try to refactor `calcExpectedValue` to somehow return two
different types of results (32 and 64-bit) depending on the relocation
type, I think we can just remove this code.

Differential Revision: https://reviews.llvm.org/D102265