build: don't rm target when cleaning

[chadoh]

The clean command was improved in #122, but it may have been made somewhat too aggressive.

I often symlink my local soroban build into target/bin/soroban. If I forget to remove target from clean, then this quickly gets deleted, which leads to unexpected behavior.

So I've been running reset over and over with target removed from the list of clean targets, and I haven't noticed any problems.

[chadoh]

@tyvdh what do you think?

[kalepail]

Fine with me if the CI build passes

[chadoh]

@tyvdh do you have the ability to merge? I do not.