Remove build for ubuntu 18.04 and macos-10.15

[fedetftpolimi]

To fix support for OpenModelica that does not expect a subdirectory with the compiler name on Linux and MacOS, now there needs to be be only one version of the library for these two OSes.
However, the CI still builds two versions for each of those OSes, and likely the second build (which happens in an unspecified order) overwrites the shared libraries of the previous build.

@jowr could you please remove from the CI the build for ubuntu 18.04 and macos-10.15, thus leaving only one build per OS.

[fedetftpolimi]

To be clear, Windows is the only OS that still needs different builds with different compilers, so the CI for Windows is already correct.
Linux and MacOS shall only have one build with the most recent version.