github: Do not test meson on Ubuntu 18.04
github: Do not test meson on Ubuntu 18.04

We are bumping Meson version to 0.53, which Ubuntu 18.04 LTS doesn't
have.  Remove the combination of meson on Ubuntu 18.04 from the
matrix.

Signed-off-by: Yasushi SHOJI <afa9ca3be149447780c2f7b524d61ed3d3db63f9@spacecubics.com>

libcsp/libcsp,libcsp/libcsp