Remove more dead code for ELF file.

`writable` is always false. `program_header_only` is always true except
for in some tests.

The code that obtains information through ElfFile with
`program_header_only=true` in tests are replaced by code that uses
libelf.

Bug: 377474232
Bug: 6527146
Test: art/test.py --host -g
Change-Id: Icc9b4dbae612a90f54b50e090e191fad0a19f709