azure: Drop 32-bit MSYS2 build

As of 2020-05-17, 32-bit MSYS2 is no longer actively supported by
the upstream [1]. Let's drop the 32-bit Windows host tool build.

[1] https://www.msys2.org/news/#2020-05-17-32-bit-msys2-no-longer-actively-supported



Signed-off-by: Bin Meng <bmeng.cn@gmail.com>