stacktrace: Remove weak version of save_stack_trace_tsk_reliable()

Recent rework of stack trace infrastructure introduced a new set of
helpers for common stack trace operations (commit e9b98e16
("stacktrace: Provide helpers for common stack trace operations") and
related). As a result, save_stack_trace_tsk_reliable() is not directly
called anywhere. Livepatch, currently the only user of the reliable
stack trace feature, now calls stack_trace_save_tsk_reliable().

When CONFIG_HAVE_RELIABLE_STACKTRACE is set and depending on
CONFIG_ARCH_STACKWALK, stack_trace_save_tsk_reliable() calls either
arch_stack_walk_reliable() or mentioned save_stack_trace_tsk_reliable().
x86_64 defines the former, ppc64le the latter. All other architectures
do not have HAVE_RELIABLE_STACKTRACE and include/linux/stacktrace.h
defines -ENOSYS returning version for them.

In short, stack_trace_save_tsk_reliable() returning -ENOSYS defined in
include/linux/stacktrace.h serves the same purpose as the old weak
version of save_stack_trace_tsk_reliable() which is therefore no longer
needed.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>