Removes redundant reduction files

[ndgrigorian]

This pull request removes two files made redundant when reductions were split out of _tensor_impl, namely, dpctl/tensor/libtensor/source/reduction_over_axis.cpp and dpctl/tensor/libtensor/source/reduction_over_axis.hpp

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• If this PR is a work in progress, are you opening the PR as a draft?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1480/index.html

[coveralls]

coverage: 85.979%. remained the same
when pulling 4060e1a on remove-unnecessary-reduction-files
into b3ba5ac on master.

[github-actions]

Array API standard conformance tests for dpctl=0.15.1dev2=py310h15de555_19 ran successfully.
Passed: 874
Failed: 57
Skipped: 59

[oleksandr-pavlyk]

Thank you @ndgrigorian !