You can't install the pgbench utilty without installing some required packages. These can be seen in the link you provided in your question:
Package: postgresql-contrib-9.3 (9.3.21-0ubuntu0.14.04 and others) [security]
Each package that has a depends (red dot) in front of it, is required to install the package you are installing.
In your case the postgresql-contrib-9.3 package depends on:
If you follow the packages you will find addtional dependencies. The postgesql-9.3 package for example depends on:
This goes on and on until all the prerequisites are met, so that your desired package can be installed.
Is there a way to only install pgbench without completely installing the PostgreSQL package which includes PostgreSQL server?
No it is not possible to install the pgbench utility contained in the postgresql-contrib-9.3 package without installing additonal packages, because of the dependencies.