libelf: There is just one ELF version.

Remove (partially defined out) code and data structures dealing with
multiple ELF versions. There hasn't been a new ELF version in the
last 20 years. Simplify the code a bit by just assuming there will
only be one version (EV_CURRENT == 1).

Simplifies elf_version, gets rid of __libelf_version_initialized.
Removes one (or more) array (version) dimension from various tables
and accessor functions (__elf_xfctstom, shtype_map, __libelf_data_type,
__libelf_type_aligns and __libelf_type_sizes).

Signed-off-by: Mark Wielaard <mark@klomp.org&gt;`>