tests: posix: common: remove duplicate call to pthread_attr_init

There was a second call to `pthread_attr_init()` that reallly had
no sense being there. Also, it seems that there was a call to
`pthread_attr_destroy()` out of perhaps paranoia.

The duplicate call and `pthread_attr_destroy()` can be removed.

Signed-off-by: Chris Friedt <cfriedt@meta.com>