Remove needless skip tests from lib/File/stat.t

Remove needless skip tests from lib/File/stat.t

The stat builtin is always implemented, so no need for an eval {} test to
check for this. The presence or absence of <sys/stat.h> at the C level isn't
going to affect the outcome of this test.

Both have been in the test script since it was added (as t/lib/filestat.t)
by commit f7a45afb043dafc5 in May 2001. I suspect that both were copied
from one of the Net::* test scripts which was being used as the template.