[llvm-readobj][test] - Remove unused Offset key from reloc-types-*.test tests

This is a follow-up for D75608.
The `Offset` property is unused and can be removed to reduce tests.

This patch does nothing with `reloc-types-elf-i386.test` which has a different
structure and kind of tests the `Offset`. I think we might want to split it probably.

Differential revision: https://reviews.llvm.org/D76195