[LLD][ELF] - Minor simplification. NFC.
This removes a call to object::getSymbol<ELFT>.
We used this function in a next way: it was given an
array of symbols and index and returned either a symbol
at the index given or a error.
It does not make much sense to keep this function on LLVM side
only for LLD, because having only a list of symbols and the index it
is not able to produce a valueable error message about context anyways.