Remove references to 'bzr' as it is no longer required

[wbh1]

Changes

After bumping the Prometheus version to 2.8.1 in #1009, bzr should no longer a build requirement. (This was addressed in prometheus/prometheus#5378). As such, it doesn't make much sense to keep it as a requirement in the Makefile anymore :)

The only remaining package with a dependency that required bzr was github.com/improbable-eng/thanos/pkg/cluster because it depended on v0.1.0 of github.com/hashicorp/memberlist. I've bumped this dependency to v0.1.3 which no longer has a dependency that needs bzr.

For historical reference, the bzr dependency was previously discussed in #868 and #948

Verification

I have verified that Thanos builds successfully still on MacOS. I could use someone to test a Linux build if they feel inclined, though. (Make sure you clean out your go mod package cache first)

All tests passed (I skipped e2e tests for store buckets, but those should not be affected).

Before, there were two modules that needed bzr installed:

go: labix.org/v2/mgo@v0.0.0-20140701140051-000000000287: bzr branch --use-existing-dir https://launchpad.net/mgo/v2 . in /Users/wbhegedus/go/pkg/mod/cache/vcs/ca61c737a32b1e09a0919e15375f9c2b6aa09860cc097f1333b3c3d29e040ea8: exec: "bzr": executable file not found in $PATH
go: launchpad.net/gocheck@v0.0.0-20140225173054-000000000087: bzr branch --use-existing-dir https://launchpad.net/~niemeyer/gocheck/trunk . in /Users/wbhegedus/go/pkg/mod/cache/vcs/f46ce2ae80d31f9b0a29099baa203e3b6d269dace4e5357a2cf74bd109e13339: exec: "bzr": executable file not found in $PATH
go: error loading module requirements

Now:

go mod why -m launchpad.net/gocheck
# launchpad.net/gocheck
(main module does not need module launchpad.net/gocheck)

go mod why -m labix.org/v2/mgo
# labix.org/v2/mgo
(main module does not need module labix.org/v2/mgo)

[brancz]

lgtm 👍 thanks!

[bwplotka]

Awesome!