[meta-pkgs/ros-base] Drop ros package alternative

Selecting among ros versions does not make much sense anymore, so drop the
corresponding package alternative and use whatever system version is
there (see comments in issue #391). System packages are searched in
/opt/ros/{noetic,melodic} only, in this order, but this can be tuned in
user configuration if needed.

Individual DEPEND_ABI.* can still be used for packages that have specific
requirements. They have be set to match any ros version more recent than
'lunar' by default.