Fixed redundant PYIMATH_EXPORTS causing compile issues on Windows Clang

[ZTKlein]

Compilation of the python bindings fails using Clang on Windows, due to declared __declspec(import)'s on code without external linkage. The behavior in MSVC is to throw these declarations away, but Clang will error out. It doesn't cause issues to remove them, as they don't serve a functional purpose -- the changed lines are in an anonymous namespace in PyImathStringTable.cpp and on deleted functions in PyImathUtil.h. Currently the PyImathTestC and PyImathTest_Python3 tests fail in my environment, though that's unrelated to the changes in this PR.

[linux-foundation-easycla]

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[ZTKlein]

Closing because I didn't have my git config set properly for the commit. Will open a new one in a bit.