VS,Xcode: Remove unused CMAKE_GENERATOR_* variables

Since the topic merged to master by commit 34a02846 (Merge topic
'ide-compiler-id', 2012-08-24), these variables are not used by
CMakeDetermine*Compiler.cmake for VS and Xcode generators.  Drop the
code that sets them.
Merge topic 'ide-compiler-vars-cleanup' into next

bed6c38 VS,Xcode: Remove unused CMAKE_GENERATOR_* variables