csi: remove unused startMockServer and github.com/rexray/gocsi dependency

[thaJeztah]

• relates to vendor: go.etcd.io/etcd v3.5.16, go.etcd.io/etcd/server/v3 v3.5.16 moby#48650

This utility was used in manager/volumes/volumes_test.go, which got deleted in b0a1aa6 (#3022), making the startMockServer unused.

Remove the file, so that we can remove the problematic github.com/rexray/gocsi dependency, which is unmaintained, and required us to use Drew's fork to prevent legacy dependencies from being pulled in through the indirect github.com/coreos/etcd/clientv3 dependency;

go: finding module for package github.com/coreos/etcd/clientv3/concurrency
go: finding module for package github.com/coreos/etcd/clientv3
go: found github.com/coreos/etcd/clientv3 in github.com/coreos/etcd v3.3.27+incompatible
go: found github.com/coreos/etcd/clientv3/concurrency in github.com/coreos/etcd v3.3.27+incompatible
go: finding module for package github.com/coreos/go-systemd/journal
go: finding module for package github.com/coreos/pkg/capnslog
go: found github.com/coreos/pkg/capnslog in github.com/coreos/pkg v0.0.0-20240122114842-bbd7aa9bf6fb
go: found github.com/coreos/go-systemd/journal in github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf

The above resolution resulted in these legacy versions to be added;

github.com/coreos/etcd v3.3.27+incompatible // indirect
github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf // indirect
github.com/coreos/pkg v0.0.0-20240122114842-bbd7aa9bf6fb // indirect

- What I did

- How I did it

- How to test it

- Description for the changelog

[thaJeztah]

@corhere @dperny PTAL

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.46%. Comparing base (911c976) to head (2501f6e).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3184      +/-   ##
==========================================
+ Coverage   57.28%   57.46%   +0.17%     
==========================================
  Files         144      144              
  Lines       30598    30598              
==========================================
+ Hits        17528    17582      +54     
+ Misses      11732    11678      -54     
  Partials     1338     1338