Remove expected failure for :coredump_phnum

The testcase no longer fails on ^/head because readelf has established parity
with binutils' copy of readelf.

This issue is not seen on Jenkins because
`test_suites.FreeBSD.allow_sysctl_side_effects` isn't set in kyua.conf on
the CI host, i.e., the test is skipped.

PR:	215019
Tested with:	binutils (amd64-binutils-2.28,1); elftoolchain (r3561M)

tests/sys/kern/coredump_phnum_test.sh

@@ -39,8 +39,6 @@ coredump_phnum_head()
 }
 coredump_phnum_body()
 {
-	atf_expect_fail "elftoolchain (base) readelf doesn't handle extended program header numbers; bug # 215019"
-
 	# Set up core dumping
 	cat > coredump_phnum_restore_state.sh <<-EOF
 	#!/bin/sh