Remove bogus requirement from libexecinfo test

The bogus requirement was causing CI infrastructure (which does not mount
procfs) to skip the test.  Procfs has not been needed by libexecinfo on
FreeBSD (nor NetBSD) for years.  Both now use a sysctl to obtain the path to
the current process image.

X-MFC-With:	r354126

contrib/netbsd-tests/lib/libexecinfo/t_backtrace.c

@@ -147,7 +147,6 @@ ATF_TC(backtrace_fmt_basic);
 ATF_TC_HEAD(backtrace_fmt_basic, tc)
 {
 	atf_tc_set_md_var(tc, "descr", "Test backtrace_fmt(3)");
-	atf_tc_set_md_var(tc, "require.files", "/proc/self");
 }
 
 ATF_TC_BODY(backtrace_fmt_basic, tc)