[FLINK-1375] [assembly] Remove incubator DISCLAIMER file from binary distribution

flink-dist/src/main/flink-bin/DISCLAIMER

-Apache Flink is an effort undergoing incubation at the Apache Software
-Foundation (ASF), sponsored by the Apache Incubator PMC.
-
-Incubation is required of all newly accepted projects until a further review
-indicates that the infrastructure, communications, and decision making process
-have stabilized in a manner consistent with other successful ASF projects.
-
-While incubation status is not necessarily a reflection of the completeness
-or stability of the code, it does indicate that the project has yet to be
-fully endorsed by the ASF.
-