[llvm-objcopy][ELF][NFC] Remove unneeded methods of Object

The patch removes mutable accessor methods for sections and segments.
As for now, const variants of them are not used because all callers have
mutable access to an instance of Object. On the other hand, they do not
actually modify the sets, so it looks better to keep only const ones.

Differential Revision: https://reviews.llvm.org/D107652