That specific warning (added with 862dc17e
"loplugin:expressionalwayszero improvements", together with other improvements)
already looked somewhat unhelpful to me in
4cb78942 "loplugin:expressionalwayszero
(clang-cl)", but now started to generate

> [CXX] vcl/source/bitmap/BitmapSobelGreyFilter.cxx
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:92:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum1 += nMask121 * nGrey12;
>                                  ^~~~~~~~~~~~~~~~~~
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:99:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum2 += nMask212 * nGrey21;
>                                  ^~~~~~~~~~~~~~~~~~
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:101:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum1 += nMask221 * nGrey22;
>                                  ^~~~~~~~~~~~~~~~~~
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:102:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum2 += nMask222 * nGrey22;
>                                  ^~~~~~~~~~~~~~~~~~
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:105:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum2 += nMask232 * nGrey23;
>                                  ^~~~~~~~~~~~~~~~~~
> /data/sbergman/lo-clang2/core/vcl/source/bitmap/BitmapSobelGreyFilter.cxx:110:34: error: expression always evaluates to zero, lhs=0 rhs=unknown [loplugin:expressionalwayszero]
>                         nSum1 += nMask321 * nGrey32;
>                                  ^~~~~~~~~~~~~~~~~~
> 6 errors generated.

(where all those nMask* are zero constants; and which even passed Gerrit/Jenkins
as loplugin:expressionalwayszero is only active for Clang >= 3.9).  Lets just
remove that specific check again.

Change-Id: Ia8710f83b16d6e6949439d3941e17b8a0959aa8b