ir: remove "is Linux string constant" property from elf_symbol

ir: remove "is Linux string constant" property from elf_symbol

This boolean property was obsoleted by the new symtab reader
implementation. It has no users.

Following this change, the find_ksymtab_strings_section function joins
find_ksymtab_section and find_ksymtab_gpl_section in having no users.

	* include/abg-ir.h (elf_symbol::elf_symbol): Drop
	is_linux_string_cst argument.
	(elf_symbol::create): Likewise.
	(elf_symbol::get_is_linux_string_cst): Drop method.
	* src/abg-dwarf-reader.cc (lookup_symbol_from_sysv_hash_tab):
	Remove code that gets the index of the __ksymtab_strings
	section. Drop corresponding elf_symbol::create argument.
	(lookup_symbol_from_gnu_hash_tab): Likewise.
	(lookup_symbol_from_symtab): Likewise.
	(create_default_fn_sym): Drop false is_linux_string_cst
	argument to elf_symbol::create.
	* src/abg-ir.cc (elf_symbol::priv::is_linux_string_cst_): Drop
	member variable.
	(elf_symbol::priv default ctor): Drop initialisation of
	is_linux_string_cst_.
	(elf_symbol::priv normal ctor): Drop is_linux_string_cst
	argument and corresponding is_linux_string_cst_
	initialisation.
	(elf_symbol::elf_symbol ctor): Drop is_linux_string_cst
	argument and corresponding forwarding to priv ctor.
	(elf_symbol::create): Drop is_linux_string_cst argument and
	corresponding forwarding to ctor.
	(elf_symbol::get_is_linux_string_cst): Drop method.
	* src/abg-reader.cc (build_elf_symbol): Drop false
	is_linux_string_cst argument to elf_symbol::create.
	* src/abg-symtab-reader.cc (symtab::load): Likewise.

Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>