[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.

This function was removed in D64631. 
(rL366052, but was reverted because of LLD compilation error
that I didn't know about).

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.

llvm-svn: 366057