`adc_lock.c` and `adc_lock.h` are removed.
Content in these two files are moved to `adc_share_hw_ctrl.c` and
`adc_share_hw_ctrl.h`, in commit 4f80c0f2.
`adc_lock.c` and `adc_lock.h` are left due to auto-solving conflicts.
This commit removes these two blank files.