Add removal function for ud_file_release_mappings_2 [RHELDST-16431]

[drepelov]

Before this MR it was possible to only add new mappings to the ud_file_mappings_2 repo note. This meant that if a file was removed from a Pulp repo, the mapping remained in the note. This MR introduces functionality to remove mappings for files that are no longer in that repo.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c876a49) 100.00% compared to head (92399ba) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #208 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 47 47 Lines 3153 3166 +13 ========================================= + Hits 3153 3166 +13 ``` | [Impacted Files](https://app.codecov.io/gh/release-engineering/pubtools-pulplib/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering) | Coverage Δ | | |---|---|---| | [pubtools/pulplib/\_impl/client/ud\_mappings.py](https://app.codecov.io/gh/release-engineering/pubtools-pulplib/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering#diff-cHVidG9vbHMvcHVscGxpYi9faW1wbC9jbGllbnQvdWRfbWFwcGluZ3MucHk=) | `100.00% <100.00%> (ø)` | |

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[rajulkumar]

Please add the description to the commit as well. If we revisit this patch in the git log, we'll only the see the header "Add removal function for ud_file_release_mappings_2 [RHELDST-16431]" that won't say much why this patch was added and will have to lookup the jira for details. Also, this being an open sourced library, probably all users won't have access to the jira.