[ELF] Remove redundant isDefined() in Symbol::computeBinding() and delete one redundant call site

After r367869, VER_NDX_LOCAL can only be assigned to Defined and
CommonSymbol.  CommonSymbol becomes Defined after replaceCommonSymbols(),
thus `versionId == VER_NDX_LOCAL` will imply `isDefined()`.

In maybeReportUndefined(), computeBinding() is called when the symbol is
unknown to be Undefined. computeBinding() != STB_LOCAL will always be
true.

llvm-svn: 368536