build: Remove execinfo.h check

The check for the execinfo.h header is only advisory; the build will not
fail if it is not present, and set HAVE_EXECINFO_H if it is. The check
was added in commit bc3e0204 ("build: Add declaration checks to check for
required syscall flags") with no obvious use or reasoning.

Remove the no-op check.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>