backtrace: Remove variant that does not support threads.

This variant was Linux-specific, GCC-specific, only worked on
architectures with frame pointers (possibly only on i386?), and isn't used
with glibc anyway.  Remove it.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>