SystemTools: Remove unused environ declaration

Commit "KWSys: Fix SystemTools environment memory handling" (2012-04-26)
added a _WIN32 case inside !KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H to dllimport
the "environ" global.  Howver, KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H is true
on every Windows toolchain we support so the case is never reached.
Furthermore, even if it were reached the use of dllimport is incorrect
because the toolchain might not be compiling with a dynamic runtime
library.  Remove the unused incorrect line and supporting conditionals.

Change-Id: If2b24fcbec18a711e4fbc28c5d027506d14f9ccb