cmake: remove policy up to 3.15
cmake: remove policy up to 3.15

Since we target CMake 3.15 all these policies are already set to NEW.

google/or-tools,or-tools/or-tools,google/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,google/or-tools,google/or-tools