I noticed that build_section_table cannot fail.  This patch changes it
to return a target_section_table and then removes the dead code.

gdb/ChangeLog
2020-10-12  Tom Tromey  <tom@tromey.com>

	* solib.c (solib_map_sections): Update.
	* record-full.c (record_full_core_open_1): Update.
	* exec.h (build_section_table): Return a target_section_table.
	* exec.c (exec_file_attach): Update.
	(build_section_table): Return a target_section_table.
	* corelow.c (core_target::core_target): Update.
	* bfd-target.c (target_bfd::target_bfd): Update.