Remove duplicate script tcp_retransmits from px/ (pixie-io#1717)

Summary: Removes duplicate script `tcp_retransmits` from
`pxl_scripts/px/`. Script already exists in `pxl_scripts/bpftrace`.
```sh
$ join -j 1 <(find px/ -type f -exec md5sum {} + | sort) <(find bpftrace/ -type f -exec md5sum {} + | sort) | awk '{print $2, $3}'
px/tcp_retransmits/manifest.yaml bpftrace/tcp_retransmits/manifest.yaml
px/tcp_retransmits/vis.json bpftrace/tcp_retransmits/vis.json

# Only difference between the version in `px` and `bpftrace` is a comment about the source, which is retained in this PR.
$ diff px/tcp_retransmits/data.pxl bpftrace/tcp_retransmits/data.pxl
/src/pxl_scripts/bpftrace/tcp_retransmits/data.pxl
102a103
>     # Source is expected to be pod, while destination is expected to be a service.
```

Type of change: /kind cleanup

Test Plan: Existing targets.

Alternative: we could also symlink them if it is important to retain
both.

Signed-off-by: Benjamin Kilimnik <bkilimnik@pixielabs.ai>

Author: benkilimnik