Remove codegen for demoted operations

[strongoier]

There's a mandatory demote_operations pass (https://github.com/taichi-dev/taichi/blob/master/taichi/transforms/demote_operations.cpp) which demotes high-level operations into low-level ones so that there's no need to handle these high-level ones in codegen of different backends. Those legacy codegen should be removed to keep the codebase clean.

• [x] pow() with an integer exponent
• [x] floordiv
• [ ] bit_shr
• [ ] BitExtractStmt