Eliminate fork_info::clobber_regs

Eliminate fork_info::clobber_regs

All fork_save_infrun_state callers pass '1' as CLOBBER_REGS nowadays.

The larger hunk in fork_save_infrun_state is just a reindentation.

gdb/ChangeLog:
2019-03-06  Pedro Alves  <palves@redhat.com>

	* linux-fork.c (fork_info::clobber_regs): Delete.
	(fork_load_infrun_state): Remove reference to 'clobber_regs'.
	(fork_save_infrun_state): Remove 'clobber_regs' parameter.  Update
	comment.  Adjust.
	(scoped_switch_fork_info::scoped_switch_fork_info)
	(checkpoint_command, linux_fork_context): Adjust
	fork_save_infrun_state calls.