Fix link-type check warning on org.apache.http.legacy.boot

There has been a link-type check warning from
org.apache.http.legacy.boot to conscrypt. The http library is built with
SDK, but the conscrypt lib isn't. Since this link-type check warnings
will turn into errors soon, we need to fix this.

LOCAL_SDK_VERSION is removed from the http library since this library is
using private APIs (TrustManagerImpl and OpenSSLContextImpl from
conscrypt.

Someone might have a concern on this because LOCAL_SDK_VERSION has been
ensuring that the lib isn't using any other private APIs. However, since
org.apache.http.legacy.boot is a runtime library and app developers are
provided with a stub library org.apache.http.legacy, which is already
being built with SDK, dropping LOCAL_SDK_VERSION from the runtime lib
won't causing any compatibility issue. The runtime lib isn't (and
shouldn't be) used for building apps.

Bug: 69899800
Test: m -j org.apache.http.legacy.boot is successful and does not show
any link-type check warning

Change-Id: Ice6ee65afe7a9cdecdda42988837fff29fba045f