VS: Fix regressed mapping for the cl `/Os` compiler flag

In commit v3.6.0-rc1~279^2~10 (VS: in Clang/C2 toolset, setup correct
compiler settings, 2016-02-18) a flag mapping was added for the clang
`-Os` flag.  However, this collides with a mapping we already had for
the MSVC flag of the same name.  This is a symptom of a larger problem
in that the VS generators need a per-toolset flag map (issue #16153).

For now, simply drop the new mapping and drop `-Os` from clang compiler
flags in the MinSizeRel configuration.

Reported-by: Felix Bruns <felixbruns@gmail.com>