The mining code in the Bitcoin core was removed in August of 2016. See the following change log:

https://bitcoin.org/en/release/v0.13.0

Before version 13, the mining code was in a file called "miner.cpp" and "miner.h".

The mining functionality that does background mining in the Bitcoin core was removed in August of 2016. See the following change log:

https://bitcoin.org/en/release/v0.13.0

The mining code itself is still available for testing and experimentation and is located in the files called "miner.cpp" and "miner.h" which are in the main src directory.