Do not load second copy of a library into same namespace

This workaround was introduced in M in order to make sure
that linker loads libraries provided in apk in case a library
with the same name was loaded by the system.

This is no longer a problem starting with Android N because
app is using different namespace and therefore does not see
libraries loaded by the system.

Test: bionic-unit-tests --gtest_filter=dl*:Dl*
Change-Id: I9995258c0f361e8df35420682b84e85a0cb924b4