Before the sender and receiver codes were split into different
repositories an "plasma" extra was added to the old, single package to
install plasma-related dependencies optionally. When the repositories
were split this extra was replicated to all packages, but in the case of
this repository this was not necessary anymore, as the plasma
functionality was never related to the sender code.

This commit removes all references to the old "plasma" extra dependency.

This work is part of YAN-1063

.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>