README: fix outdated info

[weissi]

Motivation:

The link to swift.org linked to the Swift 5 development versions but now
it's released. We also mentioned swift-nio-{http2,ssl,extras,transport-services}
but we already mention them in a more prominent place.

Modifications:

Remove outdated/duped info.

Result:

Better readme

[tomerd]

flaky test? @swift-nio-bot test this please

10:49:11 Fatal error: Posix wrapper adds more than 20% overhead (with wrapper: 0.8163410425186157, without: 0.6731261014938354: file /tmp/.swift-nio-syscall-wrappers-sh-test_myuiEq/syscallwrapper/Sources/syscallwrapper/SystemCallWrapperHelpers.swift, line 124
10:49:11 /code/IntegrationTests/tests_02_syscall_wrappers/test_01_syscall_wrapper_fast.sh: line 45: 13246 Illegal instruction     (core dumped) "$swift_binary" run -c release -Xswiftc -DRUNNING_INTEGRATION_TESTS

/cc @weissi

[weissi]

@tomerd that's a performance test which sometimes fails if the CI is running too much stuff at the same time... When we do the performance effort, this one can go out of the unit test suite...