ELF: Remove ELFT and LinkingContext template parameters from ELFReader.

Previously, ELFReader takes three template arguments: EFLT,
LinkingContextT and FileT. FileT is itself templated.
So it was a bit complicated. Maybe too much.

Most architectures don't actually need to be parameterized for ELFT.
For example, x86 is always ELF32LE and x86-64 is ELF64LE.
However, because ELFReader requires a ELFT argument, we needed
to parameterize a class even if not needed.

This patch removes the parameter from the class. So now we can
de-templatize such classes (I didn't do that in this patch, though).

This patch also removes ContextT parameter since it didn't have to be
passed as a template argument.

llvm-svn: 234853