Remove RPC interface Type

AIDL previously supported "rpc" and "flatten" types, which are not used
anywhere in the Android tree. This patch removes them, which significantly cuts
down code and complexity.

Bug: 23517584
Test: Rebuilt AOSP tree against new version
Change-Id: I6cf38d43c1ce109ffca987cc14520945aa22431f
Signed-off-by: Casey Dahlin <sadmac@google.com>