Remove useless code

The semantics of this block are such that restart will
only be false if all components of mArguments are null.
mArguments is a final array initialized in the
constructor of LegacyVpnRunner. The components of the
array are never overwritten (though the components of
those components are), and at least one of these
components will always be non-null (unless somebody
calls startLegacyVpnPrivileged with a non-existing
type, but that never happens).

For safety, this patch also adds a check so that
the code will crash early with a clear message if this
ever happens.

Test: FrameworksNetTests NetworkStackTests
Change-Id: I1f13aa08c6f1870c9610185b603af76f97c22bab