Docs do not reflect removal of `make delete`, `make unmount` options (v2.7.0+)

[xchoo]

According to the PYNQ documentation (here), it should be possible to do make delete or make unmount to delete failed build files and / or unmount images left behind from a failed build. But these options were removed in 51ab6b64, and so they no longer work.

[skalade]

We'll try to fix this in the next release.