Fix pragma usage

Fix pragma usage

https://bugs.freedesktop.org/show_bug.cgi?id=91228

Commit cdcdfe61b97a0a48ccf834b6d924d187da9609be changed two `#pragma
message` to `#pragma error` in hb-unicode.cc, however MSVC uses #error,
just like the #else branch. `#pragma error` is an unknown pragma so
MSVC does not fail the build because of it, which I believe was the
intention of that commit.

If it's meant to be an #error, then the #ifdef for _MSC_VER can be
removed entirely.