Remove unused macro variable

Remove unused variable from WESTON_PLUGIN_CALL_SINGLE macro.
We want to avoid using this macro in the compositor.
Avoid use of WESTON_PLUGIN_CALL_SINGLE in compositor

We only want to use WESTON_PLUGIN_CALL in the .c files if possible.
WESTON_PLUGIN_CALL_SINGLE is to be used internally by
WESTON_PLUGIN_CALL in compositor.h.
Add comments for WESTON_PLUGIN_* macros

Add general explanations of WESTON_PLUGIN_CALL_SINGLE and
WESTON_PLUGIN_CALL macros.