[llvm-readobj/llvm-readelf] - Simplify the code that dumps versions.

After changes introduced in D70495 and D70826 its now possible
to significantly simplify the code we have.

This also fixes an issue: previous code assumed that version strings
should always be read from the dynamic string table. While it is
normally true, the string table should be taken from the corresponding
sh_link field.

Differential revision: https://reviews.llvm.org/D70855