Remove some includes that shouldn't be needed any longer

This remove a bunch of #include statements in Scalar.cpp. I do not
think those should be needed any longer (assuming that they once
upon a time possibly were needed for legacy PM C bindings, but
that is not supported any longer).

Also removing some other #include statements not needed any longer
due to deprecation of legacy PM.

Differential Revision: https://reviews.llvm.org/D149438