CrayPrgEnv: Drop unused static library "link" flags variable

In commit c54a621b55 (Cray: New platform file for Cray Linux Environment
and PrgEnv, 2015-11-13, v3.5.0-rc1~216^2~1) we populated variable
`CMAKE_STATIC_LIBRARY_LINK_<LANG>_FLAGS`, but we never reference it, and
the generators do not use it.  Static libraries are archives, and are
not produced by a linker.