Remove isUnsafeBuild() from IpSecAlgorithm

IpSec is going to move to Connectivity mainline module so
hidden API usage should be removed. There's no formal API
could be an alternative for Build.IS_ENG. Considering
isUnsafeBuild() is not a really helpful method because it
only prints the log in eng build. Therefore, remove it.

Bug: 204153604
Test: FrameworksNetTests
Change-Id: Ic354e12423629b0d72e8087843893953a0c8c465