Remove exec_bfd_mtime define

Remove exec_bfd_mtime define

This removes the exec_bfd_mtime define, in favor of directly using the
appropriate member of the current program space.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* source-cache.c (source_cache::get_plain_source_lines): Use
	current_program_space.
	* corefile.c (reopen_exec_file): Use current_program_space.
	* exec.c (exec_file_attach): Use current_program_space.
	* exec.h (exec_bfd_mtime): Remove.