lib: remove plugin's ABI version

lib: remove plugin's ABI version

We don't need this version property.

We intend for Babeltrace 2.y to be able to load any plugin written for
Babeltrace 2.x, where y ≥ x.

With this patch, `struct __bt_plugin_descriptor` only contains the
plugin's name, because this is the only required property. Optional
properties are within an array of plugin descriptor attributes
(`struct __bt_plugin_descriptor_attribute`). Each plugin descriptor
attribute has a type (`enum __bt_plugin_descriptor_attribute_type`), so
this is how we can introduce new, optional plugin properties, instead of
relying on some ABI version.

The same goes for component class attributes
(`struct __bt_plugin_component_class_descriptor_attribute`) and their
type
(`enum __bt_plugin_component_class_descriptor_attribute_type`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4e9658f7d2216f43d872e3acd072dbdf588abe46
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2220
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>