[llvm/Object] - Remove ELFFile<ELFT>::getSection(const StringRef SectionName). NFC.

This method is dead. It was introduced in D47989,
but now the logic from D63475 is used in llvm-readobj instead.
Also it has a problem: it returns the first matching section,
even if there are multiple sections with the same name.

Differential revision: https://reviews.llvm.org/D65393

llvm-svn: 367833