Remove LAPACK lookup in LHCbMath

On (some) systems with LAPACK installed as system package, it can be found in /usr/lib64 -- and, since that directory also contains the system version of libstdc++, prepending this directory to LD_LIBRARY_PATH causes the (typically older) system libstdc++ to be found and used instead of the runtime that ships as part of LCG... causing interesting problems which show up as eg. genreflex failing to run due to missing symbols.

Here the lookup is removed as it is not needed any more.