[llvm-readobj] - Cleanup implementation LLVMStyle<ELFT>::printAddrsig().

It has following issues:
1) `getStaticSymbolName` returns `std::string`, but the code
   assigns a result to `Expected<std::string>`.
2) The code uses `unwrapOrError` and never tests the error reported.

This patch fixes these issues.

Differential revision: https://reviews.llvm.org/D87507