removed Find*.cmake scripts for which these are available in cmake itself

As per #2387, this removes find scripts for packages which have these supplied by cmake. For BLAS, Lapack, GLEW, and GSL cmake provides find scripts:

• https://cmake.org/cmake/help/v3.10/module/FindBLAS.html
• https://cmake.org/cmake/help/v3.10/module/FindLAPACK.html
• https://cmake.org/cmake/help/v3.10/module/FindGLEW.html
• https://cmake.org/cmake/help/v3.10/module/FindGSL.html

For GSL and GLEW, the cmake versions provide imported targets, (for BLAS and LAPACK only after 3.18), so this change should enable further downstream improvements of the cmake scripts that I haven't looked at yet.