Remove vendor folder during make clean

[davemay99]

Nomad 1.1.3 introduced a change in #10898 which removes the build requirement of vendored Golang dependencies.

When upgrading a source repository from a version prior to Nomad 1.1.3, if you do not remove the vendor directory before building you will get an error similar to the following:

dmay@ubu20:~/git/hashicorp/nomad$ make dev
--> Formatting HCL
==> Removing old development build...
==> Building pkg/linux_amd64/nomad with tags ui  ...
go: inconsistent vendoring in /home/dmay/git/hashicorp/nomad:
....
        To sync the vendor directory, run:
                go mod vendor
make[1]: *** [GNUmakefile:78: pkg/linux_amd64/nomad] Error 1
make: *** [GNUmakefile:244: dev] Error 2

For a smoother development experience this PR contains the following:

• Add a step to make clean to remove the vendor directory
• Add a warning to make dev to either remove the vendor directory or run make clean. NOTE -- during development some authors may need to temporarily use a vendor directory, so this does not stop the build process.

[notnoop]

The warning is sensible. LGTM.

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