Remove double-specification of default values in ParameterValue msg

[dhood]

Default values were added explicitly in #18 to prevent a valgrind error of uninitialised fields during serialisation. When messages can be created as pre-initialised with default values in client libraries, revert that PR and use default-initialised messages instead (to prevent specification of the default values in multiple locations).

Depends on ros2/ros2#396
Connects to ros2/ros2#396

[mikaelarguedas]

#18 Needs to be reverted once ros2/ros2#396 related PRs are merged.
@clalancette FYI, not sure if you want to "connect" them together

[clalancette]

@mikaelarguedas Thanks for the heads up; I'll connect this up with #396 as you suggest.