devel/capnproto: Remove BROKEN because it's actually working

There are 2 failing testcases out of 480 in kj-heavy-tests-run:

kj/async-unix-test.c++:977: failed: expected hupPromise.poll(waitScope)
[ FAIL ] async-unix-test.c++:941: UnixEventPort whenWriteDisconnected() (221 μs)
kj/async-io-test.c++:2953: failed: expected abortedPromise.poll(io.waitScope)
[ FAIL ] async-io-test.c++:2940: OS TwoWayPipe whenWriteDisconnected() (46 μs)

and this will be addressed but should not prevent us from building packages.

Scope of this seems to be limited to FreeBSD 15+ and exists at least since
capnproto 1.3+ but was actually only noticed when running the testsuite.

PR: 294259
See: https://github.com/capnproto/capnproto/issues/2594