remove launcher script

As this is a classic snap, and it is an editor with plugins,
shelling out with lot's of environment set can affect
subprocessed commands.

The only remaining clash here is the export of the QT_PLUGIN_PATH,
which is correctly set to find kate's plugins.

LD_LIBRARY_PATH and others are not needed as that is handled by use
of RPATH.

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>